Remove useless configure checks

configure.ac

@@ -29,7 +29,7 @@ AC_INIT([libmodbus],
         [http://libmodbus.org/])
 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])
+AM_INIT_AUTOMAKE([check-news foreign 1.11 silent-rules tar-pax subdir-objects])
 AC_PROG_CC
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
@@ -147,7 +147,6 @@ AC_CHECK_DECLS([TIOCSRS485], [], [], [[#include <sys/ioctl.h>]])
 # Check for RTS flags
 AC_CHECK_DECLS([TIOCM_RTS], [], [], [[#include <sys/ioctl.h>]])
 
-# Wtype-limits is not supported by gcc 4.2 (default on recent Mac OS X)
 WARNING_CFLAGS="-Wall \
 -Wmissing-declarations -Wmissing-prototypes \
 -Wnested-externs -Wpointer-arith \