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>

package/alsa-lib/alsa-lib.mk

@@ -20,7 +20,9 @@ ALSA_LIB_CONF_OPTS = \
 
 # Can't build with static & shared at the same time (1.0.25+)
 ifeq ($(BR2_STATIC_LIBS),y)
-ALSA_LIB_CONF_OPTS += --enable-shared=no
+ALSA_LIB_CONF_OPTS += \
+	--enable-shared=no \
+	--without-libdl
 else
 ALSA_LIB_CONF_OPTS += --enable-static=no
 endif