build: make alsa part of the autodetected libraries

alsa libs are already autodetected, this commit makes sure
--disable-autodetect actually disable it unless --enable-alsa is
specified.

configure

@@ -201,6 +201,7 @@ External library support:
   Also note that the following help text describes the purpose of the libraries
   themselves, not all their features will necessarily be usable by FFmpeg.
 
+  --disable-alsa           disable ALSA support [autodetect]
   --enable-avisynth        enable reading of AviSynth script files [no]
   --disable-bzlib          disable bzlib [autodetect]
   --enable-chromaprint     enable audio fingerprinting with chromaprint [no]
@@ -1481,6 +1482,7 @@ EXAMPLE_LIST="
     transcoding_example
 "
 EXTERNAL_AUTODETECT_LIBRARY_LIST="
+    alsa
     bzlib
     iconv
     libxcb
@@ -2060,7 +2062,6 @@ HAVE_LIST="
     $THREADS_LIST
     $TOOLCHAIN_FEATURES
     $TYPES_LIST
-    alsa
     atomics_native
     dos_paths
     jack
@@ -6112,8 +6113,7 @@ EOF
 fi
 check_header soundcard.h
 
-enabled_any alsa_indev alsa_outdev &&
+enabled alsa && check_lib alsa alsa/asoundlib.h snd_pcm_htimestamp -lasound
-    check_lib alsa alsa/asoundlib.h snd_pcm_htimestamp -lasound
 
 enabled jack_indev && check_lib jack jack/jack.h jack_client_open -ljack &&
     check_func jack_port_get_latency_range -ljack

libavdevice/Makefile

@@ -70,7 +70,7 @@ SKIPHEADERS-$(CONFIG_FBDEV_OUTDEV)       += fbdev_common.h
 SKIPHEADERS-$(CONFIG_LIBPULSE)           += pulse_audio_common.h
 SKIPHEADERS-$(CONFIG_V4L2_INDEV)         += v4l2-common.h
 SKIPHEADERS-$(CONFIG_V4L2_OUTDEV)        += v4l2-common.h
-SKIPHEADERS-$(HAVE_ALSA)                 += alsa.h
+SKIPHEADERS-$(CONFIG_ALSA)               += alsa.h
 SKIPHEADERS-$(HAVE_SNDIO)                += sndio.h
 
 TESTPROGS-$(CONFIG_JACK_INDEV)           += timefilter