Compile in the ALSA search path instead of relying on ld.so.conf.

Compile in the ALSA search path instead of relying on ld.so.conf.

The Ubuntu package currently uses ld.so.conf.d fragments to add
/usr/lib/alsa-lib and /usr/lib{32,64}/alsa-lib to the dlopen search path.
These don't *belong* on the global search path, and it becomes much more
problematic to put them there with the advent of multiarch because each
architecture then needs its own distinct config file to add the separate
path... which is then also put in the global library namespace.  Instead,
let ALSA make use of the already defined ALSA_PLUGIN_DIR to look up
plugins.

Signed-off-by: Jordi Mallach <jordi@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>