Add Git version string support.

Introduce a script to auto-generate version strings. Use `git describe`
to create descriptive version strings when building from a Git checkout,
use ".version" files for release tarballs.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

Makefile.am

@@ -7,3 +7,7 @@ SUBDIRS = doc po src
 EXTRA_DIST = \
 	INSTALL \
 	ABOUT-NLS
+
+BUILT_SOURCES = $(top_srcdir)/.version
+$(top_srcdir)/.version:
+	echo $(VERSION) > $@-t && mv $@-t $@