devel/creduce: add crutch for using libc++ 14 with clang++ 8

C-Reduce still depends on devel/llvm80, and this is no longer compatible
with libc++ 14 or higher, since libc++ now requires support for the
__builtin_is_constant_evaluated() builtin function (see
llvm/llvm-project@1123100
).

As a crutch, add a small header which defines a constexpr
__builtin_is_constant_evaluated() function, and include that at the top
of every compiled file, using -include, whenever the base system clang
version is 14 or higher.

Approved by:	swills (maintainer)
MFH:		2022Q1

devel/creduce/Makefile

@@ -26,7 +26,7 @@ CONFIGURE_ENV=	LLVM_CONFIG=${LOCALBASE}/bin/llvm-config${LLVM_VER} \
 		CXX=${LOCALBASE}/bin/clang++${LLVM_VER} \
 		CPP=${LOCALBASE}/bin/clang-cpp${LLVM_VER}
 
-USES=		autoreconf gmake libtool perl5
+USES=		autoreconf compiler gmake libtool perl5
 
 CFLAGS_powerpc64=	-mabi=elfv2
 
@@ -36,4 +36,12 @@ CFLAGS_powerpc64=	-mabi=elfv2
 LLD_UNSAFE=	yes
 .endif
 
+.include <bsd.port.pre.mk>
+
+# Detect base system libc++ >= 14 by checking for COMPILER_VERSION.
+# Not really the ideal way, but it should work.
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 140
+CXXFLAGS+=	-include ${FILESDIR}/builtin_is_constant_evaluated.h
+.endif
+
 .include <bsd.port.mk>

devel/creduce/files/builtin_is_constant_evaluated.h

@@ -0,0 +1 @@
+constexpr bool __builtin_is_constant_evaluated() { return false; }