math/muparser: fix build on powerpc

LLVM on powerpc doesn't have libomp:
CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)

math/muparser/Makefile

@@ -11,7 +11,7 @@ COMMENT=	Mathematical expressions parser library written in C++
 LICENSE=	BSD2CLAUSE
 LICENSE_FILE=	${WRKSRC}/License.txt
 
-USES=		cmake compiler:c++11-lang
+USES=		cmake
 USE_LDCONFIG=	yes
 
 USE_GITHUB=	yes
@@ -23,6 +23,14 @@ CMAKE_OFF=	ENABLE_SAMPLES
 
 EXAMPLES_PORTEXAMPLES=	*
 
+.include <bsd.port.options.mk>
+
+.if ${ARCH} == powerpc
+USES+=		compiler:gcc-c++11-lib
+.else
+USES+=		compiler:c++11-lang
+.endif
+
 post-install-EXAMPLES-on:
 	cd ${WRKSRC} && ${COPYTREE_SHARE} samples ${STAGEDIR}${EXAMPLESDIR}