Fixes to get it building on Mac OS X

For some reason, it was finding the Freetype in /Libraries/Framworks/Mono first, this was the only way I could fix it.

Also, I was getting:
.../magnum-plugins/src/MagnumPlugins/FreeTypeFont/FreeTypeFont.cpp:156:33: error: call to 'abs' is ambiguous

for those two lines