Makefile: propagate Git version via generated header

We set up a couple of preprocessor macros when compiling Git that
propagate the version that Git was built from to `git version` et al.
The way this is set up makes it harder than necessary to reuse the
infrastructure across the different build systems.

Refactor this such that we generate a "version-def.h" header via
`GIT-VERSION-GEN` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

version-def.h.in

@@ -0,0 +1,8 @@
+#ifndef VERSION_DEF_H
+#define VERSION_DEF_H
+
+#define GIT_VERSION "@GIT_VERSION@"
+#define GIT_BUILT_FROM_COMMIT "@GIT_BUILT_FROM_COMMIT@"
+#define GIT_USER_AGENT "@GIT_USER_AGENT@"
+
+#endif /* VERSION_DEF_H */