Add error message when building from github zip files.

Author: pavoljuhas

src/diffpy/SConscript.version

@@ -59,13 +59,23 @@ env.Append(BUILDERS={'BuildFeaturesCode' :
 
 vhpp = File('version.hpp')
 
+MY_GIT_MISSING_ERROR_MSG = """
+Cannot determine libdiffpy version.  Compile from a git repository
+or use a source archive from
+
+    https://github.com/diffpy/libdiffpy/releases
+"""
+
 # If version.hpp exists do not use git
 if os.path.isfile(str(vhpp.srcnode())):
     majorminor = parsemajorminor(vhpp.srcnode().get_contents())
 else:
     from libdiffpybuildutils import gitinfo
     vtpl = File('version.tpl')
     ginfo = gitinfo()
+    if not ginfo:
+        print MY_GIT_MISSING_ERROR_MSG
+        Exit(1)
     vhpp, = env.BuildVersionCode(['version.hpp'], vtpl)
     env.Depends(vhpp, env.Value(ginfo['version'] + ginfo['commit']))
     majorminor = (ginfo['major'], ginfo['minor'])