configure.ac: Do not enable -Wl,--no-undefined on Mac OS X.

This patch fixes this build error on Mac OS X.

  CCLD     libglapi.la
clang: warning: argument unused during compilation: '-pthread'
clang: warning: argument unused during compilation: '-pthread'
ld: unknown option: --no-undefined
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Signed-off-by: Vinson Lee <vlee@freedesktop.org>

configure.ac

@@ -336,7 +336,7 @@ dnl OpenBSD does not have DT_NEEDED entries for libc by design
 dnl so when these flags are passed to ld via libtool the checks will fail
 dnl
 case "$host_os" in
-openbsd*)
+openbsd* | darwin* )
     LD_NO_UNDEFINED="" ;;
 *)
     LD_NO_UNDEFINED="-Wl,--no-undefined" ;;