COMP: fix a link error on mac build

For unknown reasons, the Slicer-build link files included a
-F /Libraries/Frameworks command line flag, but this is not
being passed down to the extension, leading to a link error like
the one below.  Here I explicitly add the flag as a workaround
until the underlying issue is fixed.


Linking CXX shared library lib/Slicer-4.2/qt-loadable-modules/libqSlicerSlicerVmtk4Module.dylib
ld: framework not found QtSvg
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[8]: *** [lib/Slicer-4.2/qt-loadable-modules/libqSlicerSlicerVmtk4Module.dylib] Error 1
make[7]: *** [CMakeFiles/qSlicerSlicerVmtk4Module.dir/all] Error 2
make[6]: *** [all] Error 2
make[5]: *** [SlicerVmtk4/SlicerVmtk4-prefix/src/SlicerVmtk4-stamp/SlicerVmtk4-build] Error 2
make[4]: *** [SlicerVmtk4/CMakeFiles/SlicerVmtk4.dir/all] Error 2
make[3]: *** [all] Error 2
make[2]: *** [SlicerVmtk4-prefix/src/SlicerVmtk4-stamp/SlicerVmtk4-build] Error 2
make[1]: *** [CMakeFiles/SlicerVmtk4.dir/all] Error 2
make: *** [all] Error 2