Fixes syntax for FAHCORE in CMakeLists.txt.

GMX_FAHCORE triggered a set_property() call that was missing
a PROPERTY identifier and thus triggered a cmake error message.
Syntax at
http://www.cmake.org/cmake/help/v2.8.11/cmake.html#command:set_property

Change-Id: I95108ba3332e6ce5c15a4dc4bfa165c3a8e3ac8f