configure: try to support clang on netbsd

The headers on netbsd are not fully C99 compatible which leads to multiple
definitions of symbols in clang bit not with gcc.
AFAIK this has been fixed in netbsd trunk (didnt check though)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2992,6 +2992,7 @@ case $target_os in
         disable symver
         oss_indev_extralibs="-lossaudio"
         oss_outdev_extralibs="-lossaudio"
+        enabled gcc || check_ldflags -Wl,-zmuldefs
         ;;
     openbsd|bitrig)
         # On OpenBSD 4.5. the compiler does not use PIC unless