CXXFLAGS_FOR_TARGET and config/mt-gnu

[Doug Semler]

The file config/mt-gnu currently defines CXXFLAGS_FOR_TARGET =
$(CXXFLAGS) -D_GNU_SOURCE.  To me, it seems that it should be +=
-D_GNU_SOURCE (like most of the other frags).  Otherwise, I cannot
override the CXXFLAGS without overriding CXXFLAGS_FOR_TARGET.  Or am I
missing something?