alsa-lib: add patch to remove hardcoded -ldl

The AM_PATH_ALSA macro in utils/alsa.m4 unconditionally uses -ldl. This
breaks compilation of alsa-utils (and probably other packages using this
macro) for targets that do not support dynamic loading, such as for
Blackfin FLAT binaries.

This patch updates the macro to check if dlopen is available, and use that
result to conditionally add -ldl to the list of libraries.

Fixes
http://autobuild.buildroot.org/results/de2/de286880973be956f6c504aa9a758171d6f49674/build-end.log



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>