Backwards compatibility for older Xcode versions

A space after -F seems to break the build:

http://slicer.cdash.org/viewBuildError.php?buildid=76281

Indeed, man ld says:

     -Fdir       Add dir to the list of directories in which to search for
                 frameworks.  Directories specified with -F are searched in
                 the order they appear on the command line and before the
                 default search path. In Xcode4 and later, there can be a
                 space between the -F and directory.

So we need to remove the space for backward compatibility.