[cmake] Honor SKIP_INSTALL_* settings (as used in zlib). As FreeType depends on zlib, if we don't install zlib (e.g., because we defined SKIP_INSTALL_ALL), FreeType cannot be installed, too (cmake triggers an error saying that FreeType cannot be installed because zlib target isn't in the export set). * CMakeLists.txt: Honor `SKIP_INSTALL_HEADERS', `SKIP_INSTALL_LIBRARIES', and `SKIP_INSTALL_ALL' settings.