math/openmesh: fix build with GCC-based architectures

Add USES=compiler:c++11-lang.
CMake Error in src/OpenMesh/Apps/Decimating/DecimaterGui/CMakeLists.txt:

The compiler feature "cxx_decltype" is not known to CXX compiler

"GNU"

version 4.2.1.

Approved by: mentors (implicit approval)