Cygwin: Restore setting CC and CXX Makefile variables

b55e3f19 was a bit too aggressive in dropping, rather than just
un-exporting these Makefile variables.  We need to set these to the
configured host compiler if we are cross-compiling, otherwise they
default to the build compiler.

Also export CC to the mkvers.sh script (which requires it since
4eca5e6a).  It's unclear why we can't just cause windres to use the
build 'cpp' as the pre-processor there.

winsup/cygserver/Makefile.in

@@ -11,6 +11,9 @@ target_builddir:=@target_builddir@
 winsup_srcdir:=@winsup_srcdir@
 configure_args=@configure_args@
 
+CC:=@CC@
+CXX:=@CXX@
+
 CFLAGS:=@CFLAGS@
 override CXXFLAGS=@CXXFLAGS@
 override CXXFLAGS+=-MMD -Wimplicit-fallthrough=5 -Werror -D__OUTSIDE_CYGWIN__ -DSYSCONFDIR="\"$(sysconfdir)\""

winsup/cygwin/Makefile.in

@@ -14,6 +14,9 @@ target_builddir:=@target_builddir@
 winsup_srcdir:=@winsup_srcdir@
 configure_args=@configure_args@
 
+CC:=@CC@
+CXX:=@CXX@
+
 CFLAGS?=@CFLAGS@
 CXXFLAGS?=@CXXFLAGS@
 INCLUDES?=@INCLUDES@
@@ -773,6 +776,7 @@ src_files := $(foreach dir,$(VPATH),$(find_src_files))
 # second, so version.cc is always older than winver.o
 version.cc: mkvers.sh include/cygwin/version.h winver.rc $(src_files)
 	@echo "Making version.cc and winver.o";\
+	export CC="${CC}";\
 	/bin/sh ${word 1,$^} ${word 2,$^} ${word 3,$^} $(WINDRES) ${CFLAGS} -I${srcdir}/include
 $(VERSION_OFILES): version.cc
 

winsup/utils/Makefile.in

@@ -11,6 +11,9 @@ target_builddir:=@target_builddir@
 winsup_srcdir:=@winsup_srcdir@
 configure_args=@configure_args@
 
+CC:=@CC@
+CXX:=@CXX@
+
 CFLAGS_COMMON=-Wimplicit-fallthrough=4 -Werror
 CFLAGS:=@CFLAGS@
 CXXFLAGS:=@CXXFLAGS@