Makefile: Fix git usage for non-git source archive

We didn't take enough care to fix all invocations of git in this
scenario.

Fixes: 32d2739 ("Makefile: Specify version when building from tarball")
Signed-off-by: Kai Krakow <kai@kaishome.de>

lib/Makefile

@@ -1,4 +1,4 @@
-TAG := $(shell git describe --always --dirty || echo UNKNOWN)
+TAG ?= $(shell git describe --always --dirty || echo UNKNOWN)
 
 default: libcrucible.so
 %.so: Makefile