tools: suggest python2 command in configure

Try and find a suitable python2 binary and suggest it to the user in
case they start the configure script with a incompatible version.

PR-URL: #11375
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: silverwind

configure

@@ -2,7 +2,16 @@
 
 import sys
 if sys.version_info[0] != 2 or sys.version_info[1] not in (6, 7):
-  sys.stdout.write("Please use either Python 2.6 or 2.7\n")
+  sys.stderr.write('Please use either Python 2.6 or 2.7')
+
+  from distutils.spawn import find_executable as which
+  python2 = which('python2') or which('python2.6') or which('python2.7')
+
+  if python2:
+    sys.stderr.write(':\n\n')
+    sys.stderr.write('  ' + python2 + ' ' + ' '.join(sys.argv))
+
+  sys.stderr.write('\n')
   sys.exit(1)
 
 import errno