build: fallback to use autoconf 2.69 again

Unfortunately, Ubuntu, used for CI on GitHub does not have 2.70 which
is needed for `AC_PROG_CC` to test for C99.

configure.ac

@@ -5,7 +5,7 @@
 ### Base configuration
 
 # Configure autoconf
-AC_PREREQ([2.71])
+AC_PREREQ([2.69])
 
 AC_INIT([lldpd],
         [m4_esyscmd_s([./get-version])],
@@ -44,6 +44,7 @@ gl_LD_VERSION_SCRIPT
 
 # Checks for programs.
 AC_PROG_CC
+AC_PROG_CC_C99
 if test x"$ac_cv_prog_cc_c99" = x"no"; then
   AC_MSG_FAILURE([*** C99 support is mandatory])
 fi