Make hstore_plperl's build even more like plperl's

Combine the two places that set CPPFLAGS into one.  Also, some settings
should be restricted to Windows only.  More precisely, -Wno-comment is
a GCC-only option, but Windows in a makefile implies GCC at the moment.

Also, since -Wno-comment is more properly a preprocessor option, move it
to CPPFLAGS to simplify things a bit.