build-libcxx: Avoid warnings when building latest libcxxabi

build-libcxx.sh

@@ -184,7 +184,14 @@ build_all() {
         mkdir -p build-$arch-$type
         cd build-$arch-$type
         if [ "$type" = "shared" ]; then
-            LIBCXXABI_VISIBILITY_FLAGS="-D_LIBCPP_BUILDING_LIBRARY= -U_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS"
+            # Define _LIBCPP_BUILDING_LIBRARY, to an empty string to match
+            # cases in source files that also do
+            # "#define _LIBCPP_BUILDING_LIBRARY". Since Nov 3 2020, libcxxabi
+            # itself also defines this, but with a plain
+            # -D_LIBCPP_BUILDING_LIBRARY, which defines it to 1. Therefore,
+            # first undefine any potential existing define from the command
+            # line, to avoid conflicts.
+            LIBCXXABI_VISIBILITY_FLAGS="-U_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_BUILDING_LIBRARY= -U_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS"
         else
             LIBCXXABI_VISIBILITY_FLAGS=""
         fi