gmp: fix provides; enable cxx lib.
gmp: fix provides; enable cxx lib.

Begasus/haikuports,haikuports/haikuports,koletzky/haikuports,Begasus/haikuports,haikuports/haikuports,moochris/haikuports,miqlas/haikuports,miqlas/haikuports,nielx/haikuports,haikuports/haikuports,Begasus/haikuports,haikuports/haikuports,moochris/haikuports,Begasus/haikuports,moochris/haikuports,haikuports/haikuports,koletzky/haikuports,miqlas/haikuports,Begasus/haikuports,nielx/haikuports,nielx/haikuports,koletzky/haikuports