build: use LT_INIT(pic-only) instead of forcing -fPIC.

This actually allows for the user to override the decision and at the
same time supports compilers that might not be able to use -fPIC at
all.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/configure.ac b/configure.ac
index b0c0fb7..3e88a1b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,7 +13,7 @@
 AC_PROG_MKDIR_P
 AC_PROG_CC
 AC_PROG_INSTALL
-LT_INIT
+LT_INIT([pic-only])
 
 AC_CONFIG_MACRO_DIR([m4])