tools: m4: set -std=gnu17

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 <robimarko@gmail.com>