Move external GCC compiler hacks to bsd.sys.mk.

This allows respecting -nostdinc, -nostdinc++ and -nostdlib before
making the decision to add in -isystem, etc.  The -isystem flags
are problematic for building lib/libc++ and lib/libcxxrt which wants
to only use its own headers.

More information the need of these flags can be found at
https://gcc.gnu.org/ml/gcc/2016-03/msg00219.html

This also reverts r300873.

Sponsored by:	EMC / Isilon Storage Division

Makefile.inc1

@@ -527,19 +527,12 @@ TARGET_ABI=	gnueabihf
 TARGET_ABI=	gnueabi
 .endif
 .endif
-.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc
-# GCC requires -isystem and -L when using a cross-compiler.
-XCFLAGS+=	-isystem ${WORLDTMP}/usr/include -L${WORLDTMP}/usr/lib
-# Force using libc++ for external GCC.
-XCXXFLAGS+=	-isystem ${WORLDTMP}/usr/include/c++/v1 -std=c++11 \
-		-nostdinc++ -L${WORLDTMP}/../lib/libc++
-.else
+.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == "clang"
 TARGET_ABI?=	unknown
 TARGET_TRIPLE?=	${TARGET_ARCH:C/amd64/x86_64/}-${TARGET_ABI}-freebsd11.0
 XCFLAGS+=	-target ${TARGET_TRIPLE}
 .endif
 XCFLAGS+=	--sysroot=${WORLDTMP}
-.else
 .endif # ${MK_CROSS_COMPILER} == "no"
 
 .if !empty(BFLAGS)

Makefile.libcompat

@@ -72,13 +72,6 @@ LIBCOMPATCFLAGS+=	${LIBCOMPATCPUFLAGS} \
 # -B is needed to find /usr/lib32/crti.o for GCC and /usr/libsoft/crti.o for
 # Clang/GCC.
 LIBCOMPATCFLAGS+=	-B${LIBCOMPATTMP}/usr/lib${libcompat}
-.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc
-# GCC requires -isystem when using a cross-compiler.
-LIBCOMPATCFLAGS+=	-isystem ${LIBCOMPATTMP}/usr/include
-# Force using libc++ for external GCC.
-LIBCOMPATCXXFLAGS+=	-isystem ${LIBCOMPATTMP}/usr/include/c++/v1 -std=c++11 \
-			-nostdinc++ -L${LIBCOMPAT_OBJTREE}${.CURDIR}/lib/libc++
-.endif
 
 # Yes, the flags are redundant.
 LIBCOMPATWMAKEENV+= MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTREE} \

lib/libc++/Makefile

@@ -63,7 +63,7 @@ cxxrt_${_S}: ${_LIBCXXRTDIR}/${_S} .NOMETA
 .endfor
 
 WARNS=		0
-CFLAGS+=	-isystem ${HDRDIR} -isystem ${_LIBCXXRTDIR} -nostdinc++ -nostdlib -DLIBCXXRT
+CFLAGS+=	-I${HDRDIR} -I${_LIBCXXRTDIR} -nostdinc++ -nostdlib -DLIBCXXRT
 .if empty(CXXFLAGS:M-std=*)
 CXXFLAGS+=	-std=c++11
 .endif

lib/libcxxrt/Makefile

@@ -21,7 +21,7 @@ SRCS+=		libelftc_dem_gnu3.c\
 		guard.cc
 
 WARNS=		0
-CFLAGS+=	-isystem ${SRCDIR} -nostdinc++
+CFLAGS+=	-I${SRCDIR} -nostdinc++
 .if empty(CXXFLAGS:M-std=*)
 CXXFLAGS+=	-std=c++11
 .endif

share/mk/bsd.sys.mk

@@ -178,6 +178,32 @@ ACFLAGS+=	${ACFLAGS.${.IMPSRC:T}}
 CFLAGS+=	${CFLAGS.${.IMPSRC:T}}
 CXXFLAGS+=	${CXXFLAGS.${.IMPSRC:T}}
 
+# Special handling for external GCC.
+.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == "gcc"
+# GCC's --sysroot support for a cross-compiler without a default
+# TARGET_SYSTEM_ROOT does not add sysroot/usr/include in or the C++
+# include path of sysroot/usr/include/c++/v1.  They need to be added in
+# when not using -nostdinc/-nostdinc++.  This is not a problem with a
+# non-cross-compiler external GCC or the in-tree cross-compiler GCC which
+# has a default TARGET_SYSTEM_ROOT.
+.if ${CC:M--sysroot=*} || ${CFLAGS:M--sysroot=*}
+.if ${CFLAGS:M-nostdinc} == ""
+CFLAGS+=	-isystem =/usr/include
+.endif
+# Add in sysroot/usr/lib to ensure that it comes before /usr/local/lib
+# from ports compilers.
+LDFLAGS+=	-L=/usr/lib
+# We want to force building the system with our in-tree libc++.  Note that
+# this also requires a symlink in OBJDIR/lib/libc++/libstdc++.so to
+# sysroot/usr/lib/libc++.so.
+.if ${CXXFLAGS:M-nostdinc++} == "" && ${CXXFLAGS:M-nostdlib} == ""
+CXXFLAGS+=	-std=c++11 \
+		-nostdinc++ -isystem =/usr/include/c++/v1
+LDFLAGS+=	-L${OBJTOP}/lib/libc++
+.endif
+.endif	# --sysroot
+.endif	# X_COMPILER_TYPE == gcc
+
 .if defined(SRCTOP)
 # Prevent rebuilding during install to support read-only objdirs.
 .if !make(all) && make(install) && empty(.MAKE.MODE:Mmeta)