check that m4 is available before trying to use it

--HG--
extra : convert_revision : 8d4d75451fc003e3843e306008ad0632bbf0217a

ext/libelf/SConscript

@@ -92,11 +92,17 @@ if env.get('CC'):
     m4env['CC'] = env['CC']
 if env.get('CXX'):
     m4env['CXX'] = env['CXX']
+
 # If we have gm4 use it
 if subprocess.Popen("gm4 --version", shell=True, stdout=subprocess.PIPE, 
                      stderr=subprocess.STDOUT, close_fds=True).communicate()[0].find('GNU') >= 0:
     m4env['M4'] = 'gm4'
 
+# Check that m4 is available
+if not m4env.get('M4'):
+   print "Error: Can't find version of M4 macro processor. Please install M4 and try again."
+   Exit(1)
+
 m4env.Append(M4FLAGS='-DSRCDIR=%s' % Dir('.').path)
 m4env['M4COM'] = '$M4 $M4FLAGS $SOURCES > $TARGET'
 m4env.M4(target=File('libelf_convert.c'),