Stop hardcoding library names used to link with FTGL

There are still issues with using FTGL:
(At least on Linux) xbmc.bin needs to be linked with libftgl to successfully resolve functions used by ProjectM, so I believe detecting FTGL at top level would be a better solution.
Even if xbmc.bin is linked to libftgls and all symbols are resolved properly ProjectM plugin segfaults in libftgl.
I can submit a crash report if needed, but I first wanted to know if we should working towards fixing FTGL usage or it should be dropped.