Bug 1475564 - Enable libav1 when not building with msvc. r=dmajor

Differential Revision: https://phabricator.services.mozilla.com/D18028

--HG--
extra : moz-landing-system : lando

toolkit/moz.configure

@@ -425,11 +425,12 @@ imply_option('--enable-fmp4', ffmpeg, '--enable-ffmpeg')
 option('--enable-av1',
         help='Enable libaom for av1 video support')
 
-@depends('--enable-av1', target)
-def av1(value, target):
+@depends('--enable-av1', target, c_compiler)
+def av1(value, target, c_compiler):
     enabled = bool(value)
     if value.origin == 'default':
-        if target.os == 'WINNT' and target.cpu == 'x86':
+        if target.os == 'WINNT' and target.cpu == 'x86' and \
+                c_compiler and c_compiler.type == 'msvc':
             enabled = False
         else:
             enabled = True