make: Rebuild version if git commit id is changed

Original message from Tycho:

This might be a slightly controversial change since it always causes crtools.h
to be rebuilt, which may be annoying. However, right now version.h is only
generated on the initial git build and never again touched, which is also
incorrect.

We could potentially do something fancy by monitoring .git/HEAD for changes,
but since it may not always exist if building from a tarball, it is a little
sticky.

CC: Andersen <tycho.andersen@canonical.com>
CC: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

.gitignore

@@ -30,3 +30,4 @@ include/config.h
 protobuf-desc-gen.h
 criu.pc
 build
+.gitid

scripts/Makefile.version

 CRTOOLSVERSION		:= $(VERSION_MAJOR)$(if $(VERSION_MINOR),.$(VERSION_MINOR))$(if $(VERSION_SUBLEVEL),.$(VERSION_SUBLEVEL))
 
 VERSION_HEADER := include/version.h
+GITID_FILE := .gitid
 GITID := $(shell if [ -d ".git" ]; then git describe; fi)
+
 ifeq ($(GITID),)
 	GITID := 0
+else
+        GITID_FILE_VALUE := $(shell if [ `cat .gitid` = $(GITID) ]; then echo y; fi)
+        ifneq ($(GITID_FILE_VALUE),y)
+                .PHONY: $(GITID_FILE)
+        endif
 endif
 
-$(VERSION_HEADER): Makefile scripts/Makefile.version
+$(GITID_FILE):
+	$(E) "  GEN     " $@
+	$(Q) echo "$(GITID)" > $(GITID_FILE)
+
+$(VERSION_HEADER): Makefile scripts/Makefile.version $(GITID_FILE)
 	$(E) "  GEN     " $@
 	$(Q) echo "/* Autogenerated, do not edit */"			 > $(VERSION_HEADER)
 	$(Q) echo "#ifndef __CR_VERSION_H__"				>> $(VERSION_HEADER)