build: avoid adding assembler source code conditionally

Instead, create an extra library and link that one in conditionally. This
simplifies compiler language selection for the library.

close #15395
Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Thomas Guillem <thomas@gllm.fr>

modules/audio_filter/Makefile.am

@@ -57,8 +57,13 @@ libtrivial_channel_mixer_plugin_la_SOURCES = \
 libsimple_channel_mixer_plugin_la_SOURCES = \
 	audio_filter/channel_mixer/simple.c
 libsimple_channel_mixer_plugin_la_CFLAGS =
+libsimple_channel_mixer_plugin_la_LIBADD =
+
 if HAVE_NEON
-libsimple_channel_mixer_plugin_la_SOURCES += arm_neon/simple_channel_mixer.S
+EXTRA_LTLIBRARIES += libsimple_channel_mixer_plugin_arm_neon.la
+libsimple_channel_mixer_plugin_arm_neon_la_SOURCES = arm_neon/simple_channel_mixer.S
+
+libsimple_channel_mixer_plugin_la_LIBADD += libsimple_channel_mixer_plugin_arm_neon.la
 libsimple_channel_mixer_plugin_la_CFLAGS += -DCAN_COMPILE_ARM
 endif