CMake: only use -Werror=implicit-function-declaration on known compilers
CMake: only use -Werror=implicit-function-declaration on known compilers

Other compilers might not support this option.

franz/pocl,Oblomov/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,Oblomov/pocl,pocl/pocl,pocl/pocl,franz/pocl,pocl/pocl,franz/pocl,pocl/pocl