cmake HAVE_C11 examples

OpenMathLib/OpenBLAS cmake/system_check.cmake :159

CHECK_INCLUDE_FILE("stdatomic.h" HAVE_C11)

electroneum/electroneum CMakeLists.txt :1082

CHECK_C_COMPILER_FLAG(-std=c11 HAVE_C11)

ComputationalBiomechanicsLab/opensim-creator third_party/OpenBLAS/cmake/system_check.cmake :143

CHECK_INCLUDE_FILE("stdatomic.h" HAVE_C11)