buildsystem: detect new libspatialaudio

We can use the presence of the version header to detect libspatialaudio
0.4.0 or higher. (For more granular checks in the future, the defines
from the version header can be checked)

configure.ac

@@ -2966,6 +2966,15 @@ dnl  Ambisonic channel mixer and binauralizer plugin
 dnl
 PKG_ENABLE_MODULES_VLC([SPATIALAUDIO], [], [spatialaudio], [Ambisonic channel mixer and binauralizer], [auto])
 
+AS_IF([test -n ${SPATIALAUDIO_CFLAGS+set}], [
+    VLC_SAVE_FLAGS
+    AX_APPEND_FLAG([$SPATIALAUDIO_CFLAGS], [CFLAGS])
+    AC_CHECK_HEADER([spatialaudio/SpatialaudioVersion.h], [
+        AX_APPEND_FLAG([-DHAVE_SPATIALAUDIOVERSION_H=1], [SPATIALAUDIO_CFLAGS])
+    ])
+    VLC_RESTORE_FLAGS
+])
+
 dnl
 dnl  theora decoder plugin
 dnl

modules/audio_filter/meson.build

@@ -157,10 +157,16 @@ vlc_modules += {
 # Spatial audio (ambisonics/binaural) mixer
 spatial_dep = dependency('spatialaudio', required: get_option('spatialaudio'))
 if spatial_dep.found()
+    spatial_extra_cflags = []
+    if (spatial_dep.version().version_compare('>= 0.4.0'))
+        spatial_extra_cflags += ['-DHAVE_SPATIALAUDIOVERSION_H=1']
+    endif
+
     vlc_modules += {
         'name' : 'spatialaudio',
         'sources' : files('channel_mixer/spatialaudio.cpp'),
-        'dependencies' : [spatial_dep]
+        'dependencies' : [spatial_dep],
+        'c_args' : spatial_extra_cflags
     }
 endif
 
@@ -242,4 +248,3 @@ if rnnoise_dep.found()
 	'dependencies' : [m_lib, rnnoise_dep]
     }
 endif
-