games/gemrb: fix crash and improve port

• use GCC (https://github.com/gemrb/gemrb/issues/1786)
• add optional OpenGL and SDL2_mixer support
• add disabled pixelscaling to screensize option
• change GemRB.cfg into a proper conffile (remove from pkg-plist and annotate GemRB.cfg.sample with @sample)
• fix and modernize pkg-message
• use distfiles form GitHub instead of SourceForge
• make PNG, OGG, TTF and OpenAL support optional
• make optional demo assets also optional in the port
• use compiler:c++11-lang
• remove gnome and localbase uses
• remove DOC_DIR and MAN_DIR from CMAKE_ARGS
• set CMake options explicitly
• use CMake options helpers for boolean options
• organize options into AUDIO, RESOURCE and VIDEO groups

git rm games/gemrb/files/patch-gemrb_plugins_TTFImporter_CMakeLists.txt
git add games/gemrb/files/extra-patch-gemrb_plugins_TTFImporter_CMakeLists.txt