build: Improve for building the GPD version.
* configure.ac: Change how to distinguish between build types. (IS_GPD_BUILD): New ac_subst. (AC_HELP_STRING): Repalce by AS_HELP_STRING. * m4/gpg4win.m4 (GPG4WIN_RUNTIME_LIBRARY): Make libstdc++ guessing more robust. * src/gpg4win.mk.in (AUTHENTICODE_sign): Use another name for GPD. * packages/BUILDTYPE: New, * packages/Makefile.am (EXTRA_DIST): Add.