Revert "configure.ac: Use -Werror in C(PP)FLAGS"

As discussed in chat, -Werror should not be enabled by default.
Instead it should be added in our jenkins build scripts.

This reverts commit fc252ca384.

Change-Id: I92955c2f74f9a5c40e80028459100cf5e79a537b

configure.ac

@@ -53,8 +53,8 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([char foo;])],
 CFLAGS="$saved_CFLAGS"
 AC_SUBST(SYMBOL_VISIBILITY)
 
-CFLAGS+=" -Wall -Werror"
-CPPFLAGS+=" -Wall -Werror"
+CFLAGS+=" -Wall"
+CPPFLAGS+=" -Wall"
 
 AC_OUTPUT(
     libosmo-sigtran.pc