opus-build: build as C++ on Unix-like systems.
opus-build: build as C++ on Unix-like systems.

As stated in the .pro, this forces Opus symbols to be
name-mangled, thus avoiding potential clashes with
libcelt symbols.

Fixes #1096

Githlar/mumble,SuperNascher/mumble,niko20010/mumble,Githlar/mumble,niko20010/mumble,feld/mumble,SuperNascher/mumble,feld/mumble,niko20010/mumble,feld/mumble,Githlar/mumble,bheart/mumble,LuAPi/mumble,Keridos/mumble,unascribed/mumble,Lartza/mumble,Natenom/mumble,Zopieux/mumble,mbax/mumble,richard227/mumble,SuperNascher/mumble,Lartza/mumble,mbax/mumble,Natenom/mumble,Natenom/mumble,bheart/mumble,austinliou/mumble,SuperNascher/mumble,Keridos/mumble,ccpgames/mumble,niko20010/mumble,Githlar/mumble,LuAPi/mumble,Keridos/mumble,unascribed/mumble,Natenom/mumble,Keridos/mumble,Keridos/mumble,mbax/mumble,LuAPi/mumble,ccpgames/mumble,LuAPi/mumble,mbax/mumble,feld/mumble,Lartza/mumble,niko20010/mumble,Lartza/mumble,Zopieux/mumble,ccpgames/mumble,Natenom/mumble,Githlar/mumble,bheart/mumble,Zopieux/mumble,Zopieux/mumble,richard227/mumble,Lartza/mumble,richard227/mumble,SuperNascher/mumble,Githlar/mumble,richard227/mumble,feld/mumble,Lartza/mumble,Zopieux/mumble,austinliou/mumble,Githlar/mumble,unascribed/mumble,unascribed/mumble,ccpgames/mumble,LuAPi/mumble,SuperNascher/mumble,Keridos/mumble,unascribed/mumble,Natenom/mumble,Zopieux/mumble,bheart/mumble,SuperNascher/mumble,Lartza/mumble,ccpgames/mumble,feld/mumble,austinliou/mumble,richard227/mumble,niko20010/mumble,Githlar/mumble,austinliou/mumble,ccpgames/mumble,austinliou/mumble,ccpgames/mumble,LuAPi/mumble,Zopieux/mumble,feld/mumble,bheart/mumble,SuperNascher/mumble,Lartza/mumble,Natenom/mumble,unascribed/mumble,LuAPi/mumble,Keridos/mumble,Natenom/mumble,mbax/mumble,richard227/mumble,feld/mumble,unascribed/mumble,austinliou/mumble,SuperNascher/mumble,mbax/mumble,ccpgames/mumble,bheart/mumble,Keridos/mumble,niko20010/mumble,richard227/mumble,Zopieux/mumble,niko20010/mumble,bheart/mumble,LuAPi/mumble,bheart/mumble,richard227/mumble,austinliou/mumble,unascribed/mumble,austinliou/mumble,LuAPi/mumble,mbax/mumble,mbax/mumble