[Module::Build] SWIG compilation support and order of extra_compile_flags in compile_c

[R. B. <ro...@pa...>]

I submitted in ticket 17666 on rt.cpan.org a sample Build.PL that
handles SWIG compilation via subclassing, but might be better to have
this as common code part of the package. 

But one little thing I noticed in this endeavor was that for SWIG
compilation it is helpful to turn off some gcc warning messages until
such time SWIG is fixed to not cause gcc to emit them. So I changed
the order of applying user cflags (extra_compiler_flags) to come
*after* the cflags that are picked up as part of the Perl
configuration. Another possibility is to add a new option to allow
cflags specified before and afterwards.

Thanks.