Add tool to create a portable version.
* src/mkportable.c: New * src/mkportable-vanilla.h: New. * src/mkportable-light.h: New. * src/mkportable-full.h: New. * src/Makefile.am (mkportable.exe): New rule. (foosum_exe): Add mkportable.exe. * src/inst-gpg4win.nsi, src/uninst-gpg4win.nsi: Add mkportable.exe.