Add warning flags for non Visual Studio targets.

Unfortunately, check_c(xx)_compiler_flag doesnt like result names
with more than one -, meaning that testing flags with more than one
- in the name (more than one word) can't be done in a loop without
some more "clever" tricks.
Remove unreferenced variable e in the CHECK_THROWS macro.
Change to use the clang __has_attribute to check for noreturn.

The preferred way to check if clang supports noreturn is to use
__has_attribute(noreturn). Since that doesn't exist except in clang,
we need to define it to 0 for all compilers that don't have it
predefined.