Revert r232473. I have been convinced by Doug Barton and Bjoern Zeeb

that it is better to error out when people attempt to build using the
wrong bsd.*.mk files, than to silently ignore the problem.

This means, that after this commit, if you want to build kernel modules
by hand (or via a port) from a head source tree, you *must* make sure
the files in /usr/share/mk are in sync with that tree.  If that isn't
possible, for example when you are running on an older FreeBSD branch,
you can:

- Run "make buildenv" from your head source tree, to have the correct
  environment setup.  (It's advisable to have run "make buildworld", or
  at a minimum "make toolchain" first.)
- Alternatively, set MAKESYSPATH to the share/mk directory under your
  head source tree.  If your build tools are too old, other problems may
  still occur.
- Alternatively, use "make -m" and specify the share/mk directory under
  your head source tree.  Again, build tools that are too old may still
  result in trouble.

MFC after:	2 weeks

share/mk/bsd.sys.mk

@@ -28,7 +28,7 @@ CFLAGS		+= -std=${CSTD}
 . if defined(WARNS)
 .  if ${WARNS} >= 1
 CWARNFLAGS	+=	-Wsystem-headers
-.   if !defined(NO_WERROR) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !defined(NO_WERROR.clang))
+.   if !defined(NO_WERROR) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WERROR.clang))
 CWARNFLAGS	+=	-Werror
 .   endif
 .  endif
@@ -42,7 +42,7 @@ CWARNFLAGS	+=	-W -Wno-unused-parameter -Wstrict-prototypes\
 .  if ${WARNS} >= 4
 CWARNFLAGS	+=	-Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch\
 			-Wshadow -Wunused-parameter
-.   if !defined(NO_WCAST_ALIGN) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !defined(NO_WCAST_ALIGN.clang))
+.   if !defined(NO_WCAST_ALIGN) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WCAST_ALIGN.clang))
 CWARNFLAGS	+=	-Wcast-align
 .   endif
 .  endif
@@ -59,7 +59,7 @@ CWARNFLAGS	+=	-Wno-uninitialized
 CWARNFLAGS	+=	-Wno-pointer-sign
 # Clang has more warnings enabled by default, and when using -Wall, so if WARNS
 # is set to low values, these have to be disabled explicitly.
-.  if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
+.  if ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
 .   if ${WARNS} <= 3
 CWARNFLAGS	+=	-Wno-tautological-compare -Wno-unused-value\
 			-Wno-parentheses-equality -Wno-unused-function\
@@ -84,12 +84,12 @@ WFORMAT		=	1
 .  if ${WFORMAT} > 0
 #CWARNFLAGS	+=	-Wformat-nonliteral -Wformat-security -Wno-format-extra-args
 CWARNFLAGS	+=	-Wformat=2 -Wno-format-extra-args
-.   if !defined(NO_WERROR) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !defined(NO_WERROR.clang))
+.   if !defined(NO_WERROR) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WERROR.clang))
 CWARNFLAGS	+=	-Werror
 .   endif
 .  endif
 . endif
-. if defined(NO_WFORMAT) || ((${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")) && defined(NO_WFORMAT.clang))
+. if defined(NO_WFORMAT) || ((${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang") && defined(NO_WFORMAT.clang))
 CWARNFLAGS	+=	-Wno-format
 . endif
 .endif
@@ -98,7 +98,7 @@ CWARNFLAGS	+=	-Wno-format
 CWARNFLAGS	+=	-Wno-unknown-pragmas
 .endif
 
-.if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
+.if ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
 CLANG_NO_IAS	=	-no-integrated-as
 CLANG_OPT_SMALL	=	-mllvm -stack-alignment=8 -mllvm -inline-threshold=3 \
 			-mllvm -enable-load-pre=false

sys/conf/kern.mk

@@ -15,7 +15,7 @@ CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
 # Disable a few warnings for clang, since there are several places in the
 # kernel where fixing them is more trouble than it is worth, or where there is
 # a false positive.
-.if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
+.if ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
 NO_WCONSTANT_CONVERSION=	-Wno-constant-conversion
 NO_WARRAY_BOUNDS=		-Wno-array-bounds
 NO_WSHIFT_COUNT_NEGATIVE=	-Wno-shift-count-negative
@@ -50,7 +50,7 @@ CWARNEXTRA?=	-Wno-error-tautological-compare -Wno-error-empty-body \
 #                          -mno-sse3, -mno-ssse3, -mno-sse41 and -mno-sse42
 #
 .if ${MACHINE_CPUARCH} == "i386"
-.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
+.if ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
 CFLAGS+=	-mno-align-long-strings -mpreferred-stack-boundary=2 -mno-sse
 .else
 CFLAGS+=	-mno-aes -mno-avx
@@ -98,7 +98,7 @@ INLINE_LIMIT?=	15000
 # (-mfpmath= is not supported)
 #
 .if ${MACHINE_CPUARCH} == "amd64"
-.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
+.if ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
 CFLAGS+=	-mno-sse
 .else
 CFLAGS+=	-mno-aes -mno-avx

sys/conf/kern.pre.mk

@@ -34,7 +34,7 @@ _MINUS_O=	-O2
 .endif
 .endif
 .if ${MACHINE_CPUARCH} == "amd64"
-.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
+.if ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
 COPTFLAGS?=-O2 -frename-registers -pipe
 .else
 COPTFLAGS?=-O2 -pipe
@@ -84,7 +84,7 @@ INCLUDES+= -I$S/dev/cxgb -I$S/dev/cxgbe
 
 CFLAGS=	${COPTFLAGS} ${C_DIALECT} ${DEBUG} ${CWARNFLAGS}
 CFLAGS+= ${INCLUDES} -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h
-.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
+.if ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
 CFLAGS+= -fno-common -finline-limit=${INLINE_LIMIT}
 .if ${MACHINE_CPUARCH} != "mips"
 CFLAGS+= --param inline-unit-growth=100

sys/conf/kmod.mk

@@ -108,7 +108,7 @@ CFLAGS+=	-I. -I@
 # for example.
 CFLAGS+=	-I@/contrib/altq
 
-.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
+.if ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
 CFLAGS+=	-finline-limit=${INLINE_LIMIT}
 CFLAGS+= --param inline-unit-growth=100
 CFLAGS+= --param large-function-growth=1000