THRIFT-647. php: PHP library is missing install target

This patch adds a 'make install' target to a new PHP makefile. 

Patch: Anthony Molinaro

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@940325 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 3558dd8..a7ae651 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -44,6 +44,10 @@
 SUBDIRS += perl
 endif
 
+if WITH_PHP
+SUBDIRS += php
+endif
+
 # All of the libs that don't use Automake need to go in here
 # so they will end up in our release tarballs.
 EXTRA_DIST = \