Problem with "configure checks for gcc options that clang does not support"

[Tor Lillqvist]

Unfortunately, at least the Apple Clang doesn't generate any
compilation error or *proper* warning for command-line options it
doesn't understand, like -fno-enforce-eh-specs.

It just prints out clang: warning: argument unused during compilation:
'-fno-enforce-eh-specs' to stderr... Even with -Werror, this is just
printed, it doesn't cause the compilation to exit with failure.

So probably configure should check if this warning is printed to
stderr when checking what command-line options are supported?

One can get rid of the warning with the option -Qunused-arguments, but
I think that would be counter-productive. After all, if configure
thinks that the compiler supports something, even if it doesn't, that
misleading knowledge might be used to compile the code in a wrong
fashion. So it is useful to see what options are used even if they
don't have any effect.

--tml