Use CXXFLAGS instead of CFLAGS for linking C++ code

Use CXXFLAGS instead of CFLAGS for linking C++ code

Otherwise, this would break if using C and C++ compilers from
different families and they understand different options.  It already
used the right flags for compiling, this is only for linking.  Also,
the meson setup already did this correctly.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://www.postgresql.org/message-id/228700.1722717983@sss.pgh.pa.us