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]

build/Jamfile.v2

@@ -24,3 +24,5 @@ install dist-lib
     <install-type>LIB
     <location>../../../dist/lib
     ;
+
+explicit dist-lib ;