utils/alsa.m4: conditionally enable libdl in AM_PATH_ALSA m4 macro

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.

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

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>