Add support for using conditionally available NDK APIs

Coming in r26. For more, see android/ndk#837

ndk_cc_toolchain_config.bzl

@@ -532,6 +532,15 @@ def ndk_cc_toolchain_config(
             enabled = True,
         ),
 
+        # By default, allow use of APIs above the api_level, so long as the use is
+        # protected with a call to __builtin_available(android <version>, *).
+        # For more, see https://android.googlesource.com/platform/ndk/+/master/docs/BuildSystemMaintainers.md#weak-symbols-for-api-definitions
+        # This takes effect with NDK r26 and greater.
+        feature(
+            name = "android_ndk_conditional_API_availabilty",
+            enabled = True,
+        ),
+
         # User-settable feature controls warning aggressiveness for compilation.
         feature(name = "warnings_as_errors"),
 
@@ -682,6 +691,14 @@ def ndk_cc_toolchain_config(
                     ],
                     features = ["android_unwind_tables"],
                 ),
+                flag_set(
+                    actions = actions.all_compile,
+                    flags = [
+                        "-D__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__",
+                        "-Werror=unguarded-availability",
+                    ],
+                    features = ["android_ndk_conditional_API_availabilty"],
+                ),
 
                 ## Options for particular compile modes: