meson: Support auto/true/false for optional dependencies

At the moment, missing optional dependencies will be silently ignored
when the corresponding configuration flag is 'true', and won't be tested
for when the flag is 'false'.
In the autotools build, this corresponds respectively to
--enable-foo=auto and --disable-foo.
Having a way to get an error when the package is enabled but missing is
quite desirable. f52247384 has some half-baked attempt at that, but this
was supposed to be removed from the series. This causes errors when
processing meson.build, breaking the meson build.
This commit adds 'true'/'false'/'auto' in a proper way.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>

meson.build

@@ -117,22 +117,19 @@ endforeach
 #
 # Check deps which are optional but enabled by default. This foreach block only
 # checks the option, and adds the package to the deps list, while the real check
-# for the dependency is done in the foeach block below.
+# for the dependency is done in the foreach block below.
 optional_deps = [
-                    ['celt051', '>= 0.5.1.1', false, 'HAVE_CELT051'],
-                    ['opus', '>= 0.9.14', true, 'HAVE_OPUS'],
+                    ['celt051', '>= 0.5.1.1', 'HAVE_CELT051'],
+                    ['opus', '>= 0.9.14', 'HAVE_OPUS'],
                 ]
 foreach dep : optional_deps
-  if get_option(dep[0])
-    deps += [dep]
-  endif
-endforeach
-
-foreach dep : deps
-  d = dependency(dep[0], required : dep[2], version : dep[1])
-  if d.found()
-    spice_common_deps += d
-    spice_common_config_data.set(dep[3], '1')
+  option_value = get_option(dep[0])
+  if option_value != 'false'
+    d = dependency(dep[0], required: (option_value == 'true'), version : dep[1])
+    if d.found()
+      spice_common_deps += d
+      spice_common_config_data.set(dep[2], '1')
+    endif
   endif
 endforeach
 

meson_options.txt

@@ -11,13 +11,15 @@ option('extra-checks',
     description : 'Enable extra checks on code (default=false)')
 
 option('celt051',
-    type : 'boolean',
-    value : false,
-    description: 'Enable celt051 audio codec (default=false)')
+    type : 'combo',
+    choices : ['true', 'false', 'auto'],
+    value : 'auto',
+    description: 'Enable celt051 audio codec (default=auto)')
 
 option('opus',
-    type : 'boolean',
-    value : true,
+    type : 'combo',
+    choices : ['true', 'false', 'auto'],
+    value : 'true',
     description: 'Enable Opus audio codec (default=true)')
 
 option('python-checks',