TTF: reset library to NULL after destroying

prevents a potential crash on exit
Mac build: remove unwanted Min system version limitation
cmake: fix defines always being defined

PLUGINDIR, DATADIR, and SYSCONFDIR were always defined by cmake even when not wanted.
let config.h handle these and only when they have a value
Mac build (cmake): produce GemRB.app instead of gemrb.app
mac build (cmake): rpath and LIB_DIR should be the bundle Frameworks dir
mac build (cmake): plugins should be built to the bundle
mac build (cmake): libgemrb_core should be built to the bundle