'configure' does not write XXX_FOR_BUILD flags to mconfig if variables are set but empty

The configure script should always write the (possibly empty) values of CXXFLAGS_FOR_BUILD, CPPFLAGS_FOR_BUILD and LDFLAGS_FOR_BUILD to the mconfig file if they are set.

Currently it doesn't write the values if they are empty. The makefiles then fall back to using the regular (not _FOR_BUILD) flags, resulting in incorrect flags being used.