Incorrect work AX_CHECK_COMPILE_FLAG with clang

[Illia Iorin]

When I build dev-version of mesa 3d(graphics APIs) These warning messages
are appearing with clang.

warning: unknown warning option '-Wno-format-truncation'
[-Wunknown-warning-option]
They have appeared after we have added this command to our autoconf file.
AX_CHECK_COMPILE_FLAG([-Wformat-truncation], [CFLAGS="$CFLAGS
-Wno-format-truncation"])

This command works correct with gcc and g++ but with clang and clang++ this
flag is added though they don't support this flag. Does autoconf have full
clang support or autoconf has well known workaround?. Link to autoconf
file: https://gitlab.freedesktop.org/mesa/mesa/blob/master/configure.ac#L310