science/xcfun: fix build on GCC-based systems
by adding compiler:c++11-lang to USES:
CMake Error in /wrkdirs/usr/ports/science/xcfun/work/.build/CMakeFiles/CMakeTmp/CMakeLists.txt: Target "cmTC_6be99" requires the language dialect "CXX11" , but CMake does not know the compile flags to use to enable it.