Revert "Do not recompile version if not needed"

This reverts commit 529576dad6.

This feature works only with gmake, BSD systems do not have
it and we do not want to force users to install it.

src/Makefile.am

@@ -3,7 +3,10 @@
 sbin_PROGRAMS = tincd tinc sptps_test sptps_keypair
 
 ## Make sure version.c is always rebuilt with the latest git information
-$(eval include version_git.mak)
+.PHONY: ${srcdir}/version.c version_git.h
+version_git.h:
+	echo >$@
+	-(cd $(srcdir) && git describe) && echo '#define GIT_DESCRIPTION "'`(cd $(srcdir) && git describe) | sed 's/release-//'`'"' >$@
 ${srcdir}/version.c: version_git.h
 
 if LINUX

src/version_git.mak

@@ -1,9 +0,0 @@
-gitdescription_new=$(shell git describe | sed 's/^release-//')
-gitdescription_old=$(shell sed 's/.*"\(.*\)".*/\1/' version_git.h)
-
-ifneq ($(gitdescription_new), $(gitdescription_old))
-version_git.h:
-	echo $(gitdescription_new)
-	echo $(gitdescription_old)
-	echo '#define GIT_DESCRIPTION "$(gitdescription_new)"' >$@
-endif