uclibc++: fix compilation with long file paths

uclibc++: fix compilation with long file paths

Currently, uClic++ 0.2.5 fails to compile when using a long filepath.

For example, if the openwrt directory is in the path:
/tmp/this_directory_name_is_very_long/more_long_paths/.../openwrt,
then uclibc++ will cause a very obtuse error.

Although the uclibc++ makefiles do print a "File name too long" error,
it's not the final error that's printed, so it's a bit confusing:

> /bin/sh: 1:
> cannot create src/abi/libsupc/<SNIP>_libsupc++.a.dep: File name too long
> <SNIP: some other makefile output here>
> array_type_info.o: No such file or directory

Although OpenWRT 22.03 and current master branch have removed uClib++,
I thought I'd make a PR for OpenWRT 21.02, since I encountered it
and there seems to be quite a few other people experiencing the same issue.
It especially happens when using the SDK, (or when using an encrypted fs)
since the pre-packaged SDKs have very long filenames.

This patch is already in upstream [1], but has not yet been released.

[1]: https://git.busybox.net/uClibc++/commit/?id=6687fc9276fa52defaf8592f2001c19b826aec93

Signed-off-by: Alois Klink <alois@aloisklink.com>