Avoid warnings when building libcxxabi due to redefining _LIBCPP_BUILDING_LIBRARY

Some libcxxabi source files define _LIBCPP_BUILDING_LIBRARY themselves.

A "#define FOO" in a file defines it to "", while "-DFOO" on the command
line defines it to "1". Define it on the command line using "-DFOO=" to
make it match the #define within the source files.

build-libcxx.sh

@@ -134,7 +134,7 @@ 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"
+            LIBCXXABI_VISIBILITY_FLAGS="-D_LIBCPP_BUILDING_LIBRARY= -U_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS"
         else
             LIBCXXABI_VISIBILITY_FLAGS="-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS"
         fi