component/serf: explicitly use python2

This version of scons only support python2

component/serf/buildout.cfg

@@ -5,6 +5,7 @@ extends =
   ../apache/buildout.cfg
   ../util-linux/buildout.cfg
   ../openssl/buildout.cfg
+  ../python-2.7/buildout.cfg
   ../zlib/buildout.cfg
 
 [serf]
@@ -13,7 +14,7 @@ shared = true
 url = https://archive.apache.org/dist/serf/serf-1.3.9.tar.bz2
 md5sum = 370a6340ff20366ab088012cd13f2b57
 scons-command =
-  python ${scons:location}/scons.py \
+  ${python2.7:location}/bin/python ${scons:location}/scons.py \
   APR="${apr:location}" \
   APU="${apr-util:location}" \
   OPENSSL="${openssl:location}" \