Opportunistically skip building a cross-compiler with SYSTEM_COMPILER set.

This will still build the compiler for the target but will not build the
bootstrap cross-compiler in the cross-tools phase.  Other toolchain
bootstrapping, such as elftoolchan and binutils, currently still occurs.

This will utilize the default CC (cc, /usr/bin/cc) as an external compiler.

This is planned to be on-by-default eventually.

This will utilize the __FreeBSD_cc_version compiler macro defined in the
source tree and compare it to CC's version.  If they match then the
cross-compiler is skipped.  If [X]CC is an external compiler (absolute
path) or WITHOUT_CROSS_COMPILER is already set, then this logic is skipped.
If the expected bootstrap compiler type no longer matches the found CC
compiler type (clang vs gcc), then the logic is skipped.  As an extra
safety check the version number is also compared from the compiler to
the tree version.

Clang:
  The macro FREEBSD_CC_VERSION is defined in:
    lib/clang/include/clang/Basic/Version.inc
  For clang -target will be used if TARGET_ARCH != MACHINE_ARCH.  This
  is from the current external toolchain logic.  There is currently an
  assumption that the host compiler can build the TARGET_ARCH.  This
  will usually be the case since we don't conditionalize target arch
  support in clang, but it will break when introducing new
  architectures.  This problem is mitigated by incrementing the version
  when adding new architectures.

GCC:
  The macro FBSD_CC_VER is defined in:
    gnu/usr.bin/cc/cc_tools/freebsd-native.h
  For GCC there is no simple -target support when TARGET_ARCH !=
  MACHINE_ARCH.  In this case the opportunistic skip is not done.  If we
  add proper support for this case in external toolchain logic then it
  will be fine to enable.

This relies on the macros being incremented whenever any change occurs
to these compilers that warrant rebuilding files.  It also should never
repeat earlier values.

Reviewed by:	brooks, bapt, imp
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D6357

Makefile.inc1

@@ -48,6 +48,7 @@
 .error "Both TARGET and TARGET_ARCH must be defined."
 .endif
 
+SRCDIR?=	${.CURDIR}
 LOCALBASE?=	/usr/local
 
 # Cross toolchain changes must be in effect before bsd.compiler.mk
@@ -97,8 +98,59 @@ X${BINUTIL}?=	${${BINUTIL}}
 MK_CROSS_COMPILER=	no
 .endif
 
-.include <bsd.compiler.mk>		# don't depend on src.opts.mk doing it
+# Pull in COMPILER_TYPE and COMPILER_FREEBSD_VERSION early.
-.include "share/mk/src.opts.mk"	
+.include <bsd.compiler.mk>
+.include "share/mk/src.opts.mk"
+
+# Check if there is a local compiler that can satisfy as an external compiler.
+.if ${MK_SYSTEM_COMPILER} == "yes" && ${MK_CROSS_COMPILER} == "yes" && \
+    (${MK_CLANG_BOOTSTRAP} == "yes" || ${MK_GCC_BOOTSTRAP} == "yes") && \
+    !make(showconfig)
+# Which compiler is expected to be used?
+.if ${MK_CLANG_BOOTSTRAP} == "yes"
+_expected_compiler_type=	clang
+.elif ${MK_GCC_BOOTSTRAP} == "yes"
+_expected_compiler_type=	gcc
+.endif
+# If the expected vs CC is different then we can't skip.
+# GCC cannot be used for cross-arch yet.  For clang we pass -target later if
+# TARGET_ARCH!=MACHINE_ARCH.
+.if ${_expected_compiler_type} == ${COMPILER_TYPE} && \
+    (${COMPILER_TYPE} == "clang" || ${TARGET_ARCH} == ${MACHINE_ARCH})
+# It needs to be the same revision as we would build for the bootstrap.
+.if !defined(CROSS_COMPILER_FREEBSD_VERSION)
+.if ${_expected_compiler_type} == "clang"
+CROSS_COMPILER_FREEBSD_VERSION!= \
+	awk '$$2 == "FREEBSD_CC_VERSION" {printf("%d\n", $$3)}' \
+	${SRCDIR}/lib/clang/include/clang/Basic/Version.inc || echo unknown
+CROSS_COMPILER_VERSION!= \
+	awk '$$2 == "CLANG_VERSION" {split($$3, a, "."); print a[1] * 10000 + a[2] * 100 + a[3]}' \
+	${SRCDIR}/lib/clang/include/clang/Basic/Version.inc || echo unknown
+.elif ${_expected_compiler_type} == "gcc"
+CROSS_COMPILER_FREEBSD_VERSION!= \
+	awk '$$2 == "FBSD_CC_VER" {printf("%d\n", $$3)}' \
+	${SRCDIR}/gnu/usr.bin/cc/cc_tools/freebsd-native.h || echo unknown
+CROSS_COMPILER_VERSION!= \
+	awk -F. '{print $$1 * 10000 + $$2 * 100 + $$3}' \
+	${SRCDIR}/contrib/gcc/BASE-VER || echo unknown
+.endif
+.export CROSS_COMPILER_FREEBSD_VERSION CROSS_COMPILER_VERSION
+.endif	# !defined(CROSS_COMPILER_FREEBSD_VERSION)
+.if ${COMPILER_VERSION} == ${CROSS_COMPILER_VERSION} && \
+    ${COMPILER_FREEBSD_VERSION} == ${CROSS_COMPILER_FREEBSD_VERSION}
+# Everything matches, disable the bootstrap compiler.
+MK_CLANG_BOOTSTRAP=	no
+MK_GCC_BOOTSTRAP=	no
+CROSSENV+=	COMPILER_VERSION=${COMPILER_VERSION} \
+		COMPILER_TYPE=${COMPILER_TYPE} \
+		COMPILER_FREEBSD_VERSION=${COMPILER_FREEBSD_VERSION}
+.if make(buildworld)
+.info SYSTEM_COMPILER: Determined that CC=${CC} matches the source tree.  Not bootstrapping a cross-compiler.
+.endif
+.endif	# ${COMPILER_VERSION} == ${CROSS_COMPILER_VERSION}
+.endif	# ${_expected_compiler_type} == ${COMPILER_TYPE}
+.endif	# ${XCC:N${CCACHE_BIN}:M/*}
+
 
 # We must do lib/ and libexec/ before bin/ in case of a mid-install error to
 # keep the users system reasonably usable.  For static->dynamic root upgrades,
@@ -108,7 +160,6 @@ MK_CROSS_COMPILER=	no
 # This ordering is not a guarantee though.  The only guarantee of a working
 # system here would require fine-grained ordering of all components based
 # on their dependencies.
-SRCDIR?=	${.CURDIR}
 .if !empty(SUBDIR_OVERRIDE)
 SUBDIR=	${SUBDIR_OVERRIDE}
 .else

share/mk/src.opts.mk

@@ -187,7 +187,9 @@ __DEFAULT_NO_OPTIONS = \
     OPENLDAP \
     SHARED_TOOLCHAIN \
     SORT_THREADS \
-    SVN
+    SVN \
+    SYSTEM_COMPILER \
+
 
 #
 # Default behaviour of some options depends on the architecture.  Unfortunately

tools/build/options/WITHOUT_SYSTEM_COMPILER

@@ -0,0 +1,12 @@
+.\" $FreeBSD$
+Set to not opportunistically skip building a cross-compiler during the
+bootstrap phase of the build.
+Normally, if the currently installed compiler matches the planned bootstrap
+compiler type and revision, then it will not be built.
+This does not prevent a compiler from being built for installation though,
+only for building one for the build itself.
+The
+.Va WITHOUT_CLANG
+and
+.Va WITHOUT_GCC
+options control those.

tools/build/options/WITH_SYSTEM_COMPILER

@@ -0,0 +1,12 @@
+.\" $FreeBSD$
+Set to opportunistically skip building a cross-compiler during the
+bootstrap phase of the build.
+If the currently installed compiler matches the planned bootstrap compiler
+type and revision, then it will not be built.
+This does not prevent a compiler from being built for installation though,
+only for building one for the build itself.
+The
+.Va WITHOUT_CLANG
+and
+.Va WITHOUT_GCC
+options control those.