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>