linker flags merging does not handle -force_load

It it sometimes necessary to use -force_load to link in a static library, to ensure constructor functions are run, but qmake doesn't handle this when merging linker flags, resulting in stray copies of -force_load. It should behave the same as with -framework, treating -force_load and its argument as one unit. 

The workaround of setting QMAKE_LDFLAGS instead of LIBS/LIBS_PRIVATE means the library dependency is not propagated through prls.