ensure Python2 release libs are correctly found

On certain dashboards, `Python2_LIBRARY_RELEASE` var was finding
python2.6 libraries causing build failures. Fixed that.

projects/unix/python2.cmake

@@ -61,7 +61,9 @@ superbuild_add_extra_cmake_args(
   -DPython2_EXECUTABLE:FILEPATH=<INSTALL_DIR>/bin/python2.7
   -DPython2_INCLUDE_DIR:PATH=<INSTALL_DIR>/include/python2.7
   -DPython2_LIBRARY:FILEPATH=<INSTALL_DIR>/lib/libpython2.7.so
+  -DPython2_LIBRARY_RELEASE:FILEPATH=<INSTALL_DIR>/lib/libpython2.7.so
 
   -DPYTHON_EXECUTABLE:FILEPATH=<INSTALL_DIR>/bin/python2.7
   -DPYTHON_INCLUDE_DIR:PATH=<INSTALL_DIR>/include/python2.7
-  -DPYTHON_LIBRARY:FILEPATH=<INSTALL_DIR>/lib/libpython2.7.so)
+  -DPYTHON_LIBRARY:FILEPATH=<INSTALL_DIR>/lib/libpython2.7.so
+  -DPYTHON_LIBRARY_RELEASE:FILEPATH=<INSTALL_DIR>/lib/libpython2.7.so)