fix: make VERSION optional

This makes a passed-in `VERSION` eg.

    make VERSION=2.1.0 # or..
    VERSION=2.1.0 make

take precedence. The `git-describe` invocation will only occur if not
provided with a version.

Note that we need to `export` the derived `VERSION` though, otherwise
the `version` target won't "see" it and will echo "VERSION not set".

Author: wincent

Makefile

@@ -1,4 +1,5 @@
-VERSION := $(shell git describe --always --dirty)
+VERSION ?= $(shell git describe --always --dirty)
+export VERSION
 
 help:
 	@echo 'make build   - build the clipper executable'