Enable -Wall flag for compilers that support it

Add Autoconf archive macros to check for -Wall and its dependencies.
Adjust m4/.gitignore to only ignore libtool macros.

Require Autoconf 2.65 for compatibility with Autoconf archive macros.

configure.ac

@@ -1,6 +1,6 @@
 # Process this file with autoconf to produce a configure script
 
-AC_PREREQ(2.59)
+AC_PREREQ(2.65)
 AC_INIT([xrdp], [0.9.0], [xrdp-devel@googlegroups.com])
 AC_CONFIG_HEADERS(config_ac.h:config_ac-h.in)
 AM_INIT_AUTOMAKE([1.7.2 foreign])
@@ -13,6 +13,8 @@ PKG_PROG_PKG_CONFIG
 # Use silent rules by default if supported by Automake
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+AX_CFLAGS_WARN_ALL
+
 case $host_os in
 	*linux*)
 		linux=yes