bldchn, generate version.c instead of version_defs.h

Generated headers have to be in BUILT_SOURCES but it's always good if you
can avoid BUILT_SOURCES, see automake manual.
Moreover like in most of our other projects we have
EXTRA_DIST += $(BUILT_SOURCES), but version_defs.h should not be
distributed.
rename git_version_string to $(PACKAGE)_version_string
bldchn, fix indentation
Merge branch 'bld/git-version-gen' into next

* bld/git-version-gen:
  bldchn, fix indentation
  rename git_version_string to $(PACKAGE)_version_string
  bldchn, generate version.c instead of version_defs.h