Replace obsolete AC_PROG_CC_STDC by AC_PROG_CC

configure.ac

@@ -30,7 +30,7 @@ AC_INIT([libmodbus],
 AC_CONFIG_SRCDIR([src/modbus.c])
 AC_CONFIG_AUX_DIR([build-aux])
 AM_INIT_AUTOMAKE([check-news foreign 1.11 -Wall -Wno-portability silent-rules tar-pax subdir-objects])
-AC_PROG_CC_STDC
+AC_PROG_CC
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 AC_CONFIG_MACRO_DIR([m4])