build: Use LT_INIT in configure.ac.

Avoids the warning:
libtoolize: Remember to add 'LT_INIT' to configure.ac.

This is the new name for AC_PROG_LIBTOOL, so I removed that.

However to use this macro we must enable AC_USE_SYSTEM_EXTENSIONS.
(AC_GNU_SOURCE was removed back in 2011).

configure.ac

@@ -32,6 +32,8 @@ m4_define([HEADING],
 AC_CONFIG_AUX_DIR([build-aux])
 AC_REQUIRE_AUX_FILE([guestfs-test-driver])
 
+AC_USE_SYSTEM_EXTENSIONS
+
 dnl Initialize automake.
 AM_INIT_AUTOMAKE(foreign subdir-objects tar-pax) dnl NB: Do not [quote] this parameter.
 
@@ -40,6 +42,9 @@ AM_SILENT_RULES([yes]) # make --enable-silent-rules the default.
 
 AC_CONFIG_MACRO_DIR([m4])
 
+dnl Initialize libtool.
+LT_INIT
+
 dnl Stable or development version?
 BRANCH_NUMBER=libguestfs_major.libguestfs_minor
 AC_SUBST([BRANCH_NUMBER])

m4/guestfs-progs.m4

@@ -37,7 +37,6 @@ m4_ifdef([AC_PROG_SED],[
 # Define $(AWK).
 AC_PROG_AWK
 
-AC_PROG_LIBTOOL
 AC_PROG_LN_S
 
 dnl Check for cpio which isn't in the default Pardus install amazingly.