util: Add script to check gmmproc version

Hi,

This depends on glibmm MR !31 1.

From the commit message:

This adds a new utility script for Meson builds to check on what gmmproc version was used (or is to be used) to generate the sources, so that we can know whether we are ready to use compiler directives to export symbols by defining macros during the build of the various -mm C++ modules

The reasoning behind this is that modules depending on gmmproc (from the glibmm-2.4 ABI branches) may not have all the needed symbols to be marked with macros such as XXXMM_API to be annotated as gmmproc only gained such support in 2.64.0 (and more complete in 2.64.3), so we want to check for a new-enough gmmproc before we enable the build support to use __declspec(dllexport) over using gendef.exe to export symbols for the various modules in their Visual Studio builds.

With blessings, thank you!