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/src/codec/Makefile.am b/src/codec/Makefile.am
index d36e23f..665768c 100644
--- a/src/codec/Makefile.am
+++ b/src/codec/Makefile.am
@@ -3,7 +3,7 @@
 LIBVERSION=0:0:0
 
 INCLUDES = $(all_includes) -I$(top_srcdir)/include
-AM_CFLAGS = -fPIC -Wall
+AM_CFLAGS = -Wall
 
 lib_LTLIBRARIES = libosmocodec.la