buildsys: cleanup gpgme compile time check

buildsys: cleanup gpgme compile time check

- handle gpgme libs and cflags separately rather than appending to
  CFLAGS and LDFLAGS
- be consistent in AC_LINK_IFELSE check for gpgme 1.3.0 (though this is
  irrelephant since we don't actually run)
- be consistent with usage of "have" and "with" variables (this
  actually ends up reducing SLOC)
- when voluntary detection fails, unset GPGME_CFLAGS and GPGME_LIBS
- when requested support fails the version check, complain about the min
  version.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>