games/excido: Respect LDFLAGS and mark port as LLD_UNSAFE

/usr/bin/ld: error: cannot preempt symbol: alListenerf

defined in /usr/local/lib/libopenal.so
referenced by SoundCore.cpp

src/SoundCore.o:(SoundCore::SoundCore())

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p474767_s336359/logs/excido-0.1.5c_16.log

• While here fix USE_GL usage

PR: 226980
Reported by: emaste, pkg-fallout