Makefile: source RELEASE.txt when git fails

With commits 31f64b8 and 714dfbe and recently released Git v2.32 we
create a file RELEASE.txt that contains GIT_COMMIT and VERSION
variables. This should happen automatically when GitHub creates the
release archive or git-archive(1) is invoked directly.

To test, create an archive

    $ git archive --format=tar HEAD | gzip >gscloud.tar.gz

extract it and run make. The version strings produced by

    $ gscloud version

should match those for the original repository.

Fixes #131.

Author: bkircher

Makefile

@@ -1,5 +1,12 @@
-VERSION=$$(git describe --tags)
-GIT_COMMIT=$$(git rev-list -1 HEAD)
+VERSION := $(shell if ! git describe --tags 2>/dev/null; then \
+	grep -Po '(?<=^VERSION=)v.*$$' $$PWD/RELEASE.txt; \
+fi; \
+)
+
+GIT_COMMIT := $(shell if ! git rev-list -1 HEAD 2>/dev/null; then \
+	grep -Po '(?<=^GIT_COMMIT=)\w*$$' $$PWD/RELEASE.txt; \
+fi; \
+)
 
 default: build