Issue #457 - Silence some GCC compiler warnings in FFmpeg code

dom/media/platforms/ffmpeg/ffmpeg57/moz.build

@@ -15,7 +15,10 @@ LOCAL_INCLUDES += [
 ]
 
 if CONFIG['GNU_CXX']:
-  CXXFLAGS += [ '-Wno-deprecated-declarations' ]
+  CXXFLAGS += [
+    '-Wno-attributes',
+    '-Wno-deprecated-declarations',
+  ]
 if CONFIG['CLANG_CXX']:
   CXXFLAGS += [
     '-Wno-unknown-attributes',

dom/media/platforms/ffmpeg/ffmpeg58/moz.build

@@ -15,7 +15,10 @@ LOCAL_INCLUDES += [
 ]
 
 if CONFIG['GNU_CXX']:
-  CXXFLAGS += [ '-Wno-deprecated-declarations' ]
+  CXXFLAGS += [
+    '-Wno-attributes',
+    '-Wno-deprecated-declarations',
+  ]
 if CONFIG['CLANG_CXX']:
   CXXFLAGS += [
     '-Wno-unknown-attributes',

dom/media/platforms/ffmpeg/ffvpx/moz.build

@@ -28,7 +28,10 @@ if CONFIG['OS_ARCH'] == 'WINNT':
     ]
 
 if CONFIG['GNU_CXX']:
-  CXXFLAGS += [ '-Wno-deprecated-declarations' ]
+  CXXFLAGS += [
+    '-Wno-attributes',
+    '-Wno-deprecated-declarations',
+  ]
 if CONFIG['CLANG_CXX']:
   CXXFLAGS += [
     '-Wno-unknown-attributes',

dom/media/platforms/ffmpeg/libav55/moz.build

@@ -15,7 +15,10 @@ LOCAL_INCLUDES += [
 ]
 
 if CONFIG['GNU_CXX']:
-  CXXFLAGS += [ '-Wno-deprecated-declarations' ]
+  CXXFLAGS += [
+    '-Wno-attributes',
+    '-Wno-deprecated-declarations',
+  ]
 if CONFIG['CLANG_CXX']:
   CXXFLAGS += [
     '-Wno-unknown-attributes',

media/ffvpx/libavcodec/moz.build

@@ -3,7 +3,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-# Due to duplicate file names, we compile libavutil/x86 in its own
+# Due to duplicate file names, we compile libavcodec/x86 in its own
 # moz.build file.
 if CONFIG['FFVPX_ASFLAGS']:
     DIRS += ['x86']
@@ -68,6 +68,9 @@ SOURCES += [
 SYMBOLS_FILE = 'avcodec.symbols'
 NO_VISIBILITY_FLAGS = True
 
+if CONFIG['GNU_CXX']:
+  CXXFLAGS += ['-Wno-attributes']
+
 # GCC 10 defaults -fno-common, we don't care to solve this "properly" yet
 # so use GCC < 10 behavior.
 if CONFIG['GNU_CC'] and CONFIG['CC_VERSION'] >= '10.0.0':