Make MK_GNUCXX mean "build the libstdc++ and libsupc++ libraries" and

nothing more. Force it to be "no" when MK_CXX is "no" to simplify
usage.  It no longer also means "build g++" since we no longer have a
platform where that's interesting now that pc98 no longer needs clang
and gcc, but not g++. pc98 now just uses clang after boot2 changes.

gnu/lib/Makefile

@@ -14,7 +14,7 @@ SUBDIR+= tests
 
 # libsupc++ uses libstdc++ headers, although 'make includes' should
 # have taken care of that already.
-.if ${MK_GNUCXX} != "no" && ${MK_CXX} != "no"
+.if ${MK_GNUCXX} != "no"
 SUBDIR+= libstdc++ libsupc++
 .endif
 

gnu/usr.bin/cc/Makefile

@@ -12,12 +12,7 @@ SUBDIR+= cpp
 .endif
 
 .if ${MK_CXX} != "no"
-.if ${MK_GNUCXX} != "no"
-SUBDIR+= cc1plus c++ 
-.endif
-# This should be moved into the above block once c++filt from elftoolchain or
-# similar is provided.
-SUBDIR+= c++filt
+SUBDIR+= cc1plus c++ c++filt
 .endif
 
 .if ${MK_GCOV} != "no"