configure: uClibc native pthread requires -ldl

In the configure script, add a pthread detection with
-ldl added to cflags, because uClibc requires -ldl to
link with native libpthread.

Tested with a custom ARM toolchain with uClibc 0.9.33.2 and gcc 4.6.3.
Signed-off-by: Neil Armstrong <narmstrong@neotion.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -4593,6 +4593,9 @@ if ! disabled pthreads && ! enabled w32threads && ! enabled os2threads; then
     elif check_func pthread_join -pthreads && check_func pthread_create -pthreads; then
         add_cflags -pthreads
         add_extralibs -pthreads
+    elif check_func pthread_join -ldl -pthread && check_func pthread_create -ldl -pthread; then
+        add_cflags -ldl -pthread
+        add_extralibs -ldl -pthread
     elif check_func pthread_join -lpthreadGC2 && check_func pthread_create -lpthreadGC2; then
         add_extralibs -lpthreadGC2
     elif check_lib pthread.h pthread_join -lpthread && check_lib pthread.h pthread_create -lpthread; then