configure: disable -z defs linker check for FreeBSD.

When linking libvlccore with -z defs, the following happens:

posix/.libs/netconf.o: In function `vlc_getProxyUrl': ./src/posix/netconf.c:75:
undefined reference to `environ'

environ on FreeBSD is defined only for the dynamically linked
executables via csu, and is placed in bss section.  It's not available in
libc, which uses the same extern char **environ to access it.

What's interesting is NetBSD took a more compatible approach, where
environ is available both in libc and in csu.

configure.ac

@@ -118,6 +118,7 @@ case "${host_os}" in
     CFLAGS="${CFLAGS} -pthread"
     CPPFLAGS="${CPPFLAGS} -I/usr/local/include"
     LDFLAGS="${LDFLAGS} -L/usr/local/lib"
+    ac_cv_ld_z_defs=no
     ;;
   openbsd*)
     SYS=openbsd