After r232322, it turned out many people (and some ports) are building

kernel modules using their old installed /usr/share/mk/bsd.*.mk files,
instead of the updated ones in their source tree.  This leads to errors
like:

  "sys/conf/kmod.mk", line 111: Malformed conditional (${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang")

Obviously, these errors will go away after a "make installworld", or
alternatively, by using "make buildenv" before attempting to manually
build modules.

However, since it is apparently an expected use case to build using old
.mk files, change the way we test for clang, so it also works when the
MK_CLANG_IS_CC macro doesn't exist.

Note the conditional expressions are becoming rather unreadable now, but
I will attempt to fix that on a followup commit.

MFC after:	2 weeks

share/mk/bsd.sys.mk

@@ -28,7 +28,7 @@ CFLAGS		+= -std=${CSTD}
 . if defined(WARNS)
 .  if ${WARNS} >= 1
 CWARNFLAGS	+=	-Wsystem-headers
-.   if !defined(NO_WERROR) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WERROR.clang))
+.   if !defined(NO_WERROR) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !defined(NO_WERROR.clang))
 CWARNFLAGS	+=	-Werror
 .   endif
 .  endif
@@ -42,7 +42,7 @@ CWARNFLAGS	+=	-W -Wno-unused-parameter -Wstrict-prototypes\
 .  if ${WARNS} >= 4
 CWARNFLAGS	+=	-Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch\
 			-Wshadow -Wunused-parameter
-.   if !defined(NO_WCAST_ALIGN) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WCAST_ALIGN.clang))
+.   if !defined(NO_WCAST_ALIGN) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !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 ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
+.  if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
 .   if ${WARNS} <= 3
 CWARNFLAGS	+=	-Wno-tautological-compare -Wno-unused-value\
 			-Wno-parentheses-equality -Wno-unused-function\
@@ -84,12 +84,12 @@ WFORMAT		=	1
 .  if ${WFORMAT} > 0
 #CWARNFLAGS	+=	-Wformat-nonliteral -Wformat-security -Wno-format-extra-args
 CWARNFLAGS	+=	-Wformat=2 -Wno-format-extra-args
-.   if !defined(NO_WERROR) && ((${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang") || !defined(NO_WERROR.clang))
+.   if !defined(NO_WERROR) && ((${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")) || !defined(NO_WERROR.clang))
 CWARNFLAGS	+=	-Werror
 .   endif
 .  endif
 . endif
-. if defined(NO_WFORMAT) || ((${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang") && defined(NO_WFORMAT.clang))
+. if defined(NO_WFORMAT) || ((${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")) && defined(NO_WFORMAT.clang))
 CWARNFLAGS	+=	-Wno-format
 . endif
 .endif
@@ -98,7 +98,7 @@ CWARNFLAGS	+=	-Wno-format
 CWARNFLAGS	+=	-Wno-unknown-pragmas
 .endif
 
-.if ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
+.if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
 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 ${MK_CLANG_IS_CC} != "no" || ${CC:T:Mclang} == "clang"
+.if ${CC:T:Mclang} == "clang" || (defined(MK_CLANG_IS_CC) && ${MK_CLANG_IS_CC} != "no")
 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 ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
+.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
 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 ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
+.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
 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 ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
+.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
 COPTFLAGS?=-O2 -frename-registers -pipe
 .else
 COPTFLAGS?=-O2 -pipe
@@ -84,7 +84,7 @@ INCLUDES+= -I$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 ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
+.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
 CFLAGS+= -fno-common -finline-limit=${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 ${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang"
+.if ${CC:T:Mclang} != "clang" && (!defined(MK_CLANG_IS_CC) || ${MK_CLANG_IS_CC} == "no")
 CFLAGS+=	-finline-limit=${INLINE_LIMIT}
 CFLAGS+= --param inline-unit-growth=100
 CFLAGS+= --param large-function-growth=1000