contrib: add MinGW cross-compilation instructions
With these instructions it is possible to generate a libsipe.dll on a
Linux machine, that is compatible with the official Pidgin Windows
release.
Cleaned up src/core/Makefile.mingw:
- removed gmime, we're only compiling against libpurple anyway
- removed libiconv, it doesn't seem to be needed anymore
- added support for MinGW cross-compilation build
Updated several Makefile.am's to ensure that all source code gets
included in the distribution archive.