build: drop obsolete autoconf macro

build: drop obsolete autoconf macro

AC_HEADER_STDC is deprecated and should be replaced with
AC_CHECK_INCLUDES_DEFAULT. We haven't done this until now because the
latter is only available since autoconf v2.71. We kept supporting v2.69
for the sake of Debian stable but it has since upgraded to autoconf v2.71
so we update configure.ac as well and fix an autoconf warning.

Link: https://lore.kernel.org/r/20241003135806.41934-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>