math/cln: fix build on GCC architectures

Use new GCC to fix build after last update:
../include/cln/number.h: In function 'const type& cln::the(const cln::cl_number&)':
../include/cln/number.h:261: error: there are no arguments to 'static_assert' that depend on a template parameter, so a declaration of 'static_assert' must be available
../include/cln/number.h:261: error: (if you use '-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated)