math/csdp: fix build on powerpc

LLVM on powerpc doesn't have libomp:
cc -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing  -fopenmp -ansi -Wall -DBIT64 -DUSEOPENMP -DSETNUMTHREADS -DUSESIGTERM -DUSEGETTIME -I../include   -c -o op_o.o op_o.c
op_o.c:15:10: fatal error: 'omp.h' file not found

math/csdp/Makefile

@@ -29,6 +29,12 @@ PLIST_FILES=	bin/complement \
 		bin/rand_graph \
 		bin/theta
 
+.include <bsd.port.options.mk>
+
+.if ${ARCH} == powerpc
+USES+=		compiler:gcc-c++11-lib
+.endif
+
 post-stage:
 	@${STRIP_CMD} ${STAGEDIR}${PREFIX}/bin/*