Bug 1962300 - Provide support for unavailable symbols as weak symbols r=m_kato,gfx-reviewers,firefox-build-system-reviewers,glandium,necko-reviewers,geckoview-reviewers,cubeb-reviewers,valentin,padenot,jnicol

Compiler support make it possible to detect missing checks at compile
time, so it's relatively safe, as described in https://developer.android.com/ndk/guides/using-newer-apis

It's now applied globally on android.

Deactivate the associate compiler warning on cubeb as it otherwise
throws warning for type computations.

Differential Revision: https://phabricator.services.mozilla.com/D246803

build/moz.configure/android-ndk.configure

@@ -274,11 +274,29 @@ def extra_toolchain_flags(android_sysroot, toolchain_dir):
 
 @depends(extra_toolchain_flags)
 def android_flags(extra_toolchain_flags):
+    wead_symbols_defines = [
+        "-D__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__",
+    ]
+    weak_symbols_flags = wead_symbols_defines + [
+        "-Werror=unguarded-availability",
+    ]
     return namespace(
-        cflags=extra_toolchain_flags + ["-fno-short-enums"],
-        cxxflags=extra_toolchain_flags + ["-fno-short-enums"],
+        cflags=extra_toolchain_flags
+        + [
+            "-fno-short-enums",
+        ]
+        + weak_symbols_flags,
+        cxxflags=extra_toolchain_flags
+        + [
+            "-fno-short-enums",
+        ]
+        + weak_symbols_flags,
         ldflags=extra_toolchain_flags,
-        asflags=extra_toolchain_flags + ["-DANDROID"],
+        asflags=extra_toolchain_flags
+        + [
+            "-DANDROID",
+        ]
+        + wead_symbols_defines,
     )