games/gemrb: USE_GCC again to fix crash
- readd USE_GCC=yes (https://github.com/gemrb/gemrb/issues/2074)
- use compiler:c++14-lang (per gemrb 0.9.3 release notes)
- update pkg-plist (two files are depending on OPENGL option now)
- mention USE_TEST and USE_TRACY build options for completeness