Fix macOS compilation for fizzy-unittests for floating points

[axic]

[ 48%] Building CXX object test/unittests/CMakeFiles/fizzy-unittests.dir/execute_floating_point_conversion_test.cpp.o
In file included from /fizzy/test/unittests/execute_floating_point_conversion_test.cpp:6:
/fizzy/test/unittests/execute_floating_point_test.hpp:16:14: error: '#pragma FENV_ACCESS' is not supported on this target - ignored [-Werror,-Wignored-pragmas]
#pragma STDC FENV_ACCESS ON
             ^
1 error generated.
make[2]: *** [test/unittests/CMakeFiles/fizzy-unittests.dir/execute_floating_point_conversion_test.cpp.o] Error 1
make[1]: *** [test/unittests/CMakeFiles/fizzy-unittests.dir/all] Error 2