Use setup.py to obtain version value for scons.

Update egg info when version string changes.
Do not use git internals, the package may be built without git.

Author: pavoljuhas

srrealmodule/SConscript

@@ -19,11 +19,18 @@ module = env.SharedLibrary('srreal_ext', GlobSources('*.cpp'),
         SHLIBPREFIX='', SHLIBSUFFIX='.so')
 Alias('module', module)
 
-# install in a development mode
-gitdir = env['ENV'].get('GIT_DIR', '#/.git')
-gheads = Glob(gitdir + '/HEAD') + Glob(gitdir + '/refs/heads/*')
-egginfo = env.Command(NoCache('#/diffpy.srreal.egg-info/PKG-INFO'), gheads,
-        '$python setup.py egg_info')
+# update egg info when package version changes.
+basedir = Dir('#').abspath
+version = pyoutput(
+    'import sys\n'
+    'sys.path.insert(0, %r)\n'
+    'from setup import versiondata\n'
+    'print(versiondata.get("DEFAULT", "version"))\n' % basedir)
+egginfo = env.Command(NoCache('#/diffpy.srreal.egg-info/PKG-INFO'),
+                      env.Value(version),
+                      '$python setup.py egg_info')
+
+# install extension module in a development mode.
 develop = Alias('develop', [egginfo, Install('#/diffpy/srreal', module)])
 
 test = env.Alias('test', develop,