tools: m4: set -std=gnu17
Fedora 42 updated to GCC15 which now defaults to GNU23 as the default
instead of GNU17[1], and this breaks m4 compilation.
Its been reported upstream [2], so until its fixed lets simply set C
language version back to GNU17.
[1] https://gcc.gnu.org/gcc-15/porting_to.html#c23
[2] https://savannah.gnu.org/support/?111150
Link: https://github.com/openwrt/openwrt/pull/18505
Signed-off-by: Robert Marko <[email protected]>
Robert Marko