Add --build-libversions option and extract version numbers from GIR

This commit
 * adds an option 'build-libversions' that enables the user to specify the
   library versions, for which to create bidnings. This enables creating
   bindings that expose version-correct API.
 * adds a XSLT preprocessing step, in which all version numbers of library
   API elements are extracted. Those version numbers are intended to be used
   for the generation of DEFINE flags to enable conditional builds of the
   binding. Builds will be created only vor versions that are specified in the
   'build-libversions' argument.

NOTE: The generation of DEFINE flags will be implemented in a future commit.
Remove superfluous 'all' target Makefile.am.template