Makefile: detect version properly from dirname.

Our reproducibile builds use the dirname to get version, but they have
a v in them (the tools/repro-build.sh script gets this right, so I
copied that).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -1,7 +1,7 @@
 #! /usr/bin/make
 
 # Extract version from git, or if we're from a zipfile, use dirname
-VERSION=$(shell git describe --always --dirty=-modded --abbrev=7 2>/dev/null || pwd | sed -n 's,.*/lightning-\([0-9.rc]*\)$$,\1,p')
+VERSION=$(shell git describe --always --dirty=-modded --abbrev=7 2>/dev/null || pwd | sed -n 's,.*/clightning-\(v[0-9.rc]*\)$$,\1,p')
 
 ifeq ($(VERSION),)
 $(error "ERROR: git is required for generating version information")