alsa-lib: disable libdl for static only builds

-ldl includes the reference to the library that has
the symbols for loading dynamic libraries.
So there is no need for this library for static only builds.
Even though alsa-lib uses dlopen, it has functions snd_dlopen
and others which can emulate dynamic linking for the static
build of the alsa-lib library and there is no need in libdl
dlopen when static build only.
This patch fixes build failures of the shairport-sync
(and probably other packages) when static libs are used.

Fixes:
http://autobuild.buildroot.net/results/85a/85a3f01bcd4eb2ce7ac480e63b4cc04bf7c8f3e5/



[Thomas: use a single ALSA_LIB_CONF_OPTS for both --enable-shared=no
and --without-libdl.]

Signed-off-by: Lada Trimasova <ltrimas@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>