build: use LT_INIT([pic-only]) instead of forcing -fPIC in AM_CFLAGS.

Also contextually remove the AC_PROG_LIBTOOL call (which is an
obsolete name for what is now LT_INIT).

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/configure.ac b/configure.ac
index f90da49..5be721f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11,8 +11,7 @@
 AC_PROG_MAKE_SET
 AC_PROG_CC
 AC_PROG_INSTALL
-LT_INIT
-AC_PROG_LIBTOOL
+LT_INIT([pic-only])
 
 AC_CONFIG_MACRO_DIR([m4])