math/clasp: fix build on GCC architectures

Use C++11 compiler:
CMake Error in app/CMakeLists.txt:
  Target "clasp" requires the language dialect "CXX11" (with compiler
  extensions), but CMake does not know the compile flags to use to enable it.

MFH:		2020Q3 (fix build blanket)

math/clasp/Makefile

@@ -14,7 +14,7 @@ LICENSE_FILE=	${WRKSRC}/LICENSE
 
 BROKEN_sparc64=	Fails to build: internal compiler error
 
-USES=	cmake
+USES=	compiler:c++11-lang cmake
 
 USE_GITHUB=	yes
 GH_ACCOUNT=	potassco