[gtkada] improvements to the GtkAda build system

[Arnaud Charlet]

> Here's the disable-static patch again, unchanged since last time as it
> still applies to the trunk.
> 
> * aclocal.m4 (configure, BUILD_STATIC):
> Add the options --enable-static and --disable-static to allow building
> only
> shared libraries.
> * Makefile.common.in, Makefile.in (make, make install):
> Build and install static libraries only if BUILD_STATIC is "yes".

Thanks, patch committed to trunk.

Arno