Set ALLOW_UNKNOWN_WARNING_OPTION

header-checker build may use an older version of clang than the platform
version, causing errors for unknown cflags. Setting the env var allows
these errors to be suppressed.

Test: presubmit
Change-Id: Icf08ec5913f5608ffc1e14daf3e6ed71610f733a

Author: kongy

vndk/tools/header-checker/android/build-prebuilts.sh

@@ -118,6 +118,10 @@ cat > "${SOONG_OUT}/soong.variables" << __EOF__
 }
 __EOF__
 
+# Allow unknown warning options since this may lag behind platform's compiler
+# version.
+export ALLOW_UNKNOWN_WARNING_OPTION=true
+
 binaries=()
 for name in "${SOONG_BINARIES[@]}"; do
     binaries+=("${SOONG_HOST_OUT}/bin/${name}")