Make install target explicit. Otherwise top-level Jamfile will try to

install both results of 'boost_program_options' and of the install target,
resulting in duplicate target error.


[SVN r29604]

libs/program_options

@@ -1 +1 @@
-Subproject commit f88e0a95721cc0dfc0874a5a84ebb88bb28a2339
+Subproject commit de511c601abc591ea646a9ab30102cc3087641ca