[PATCH] Makefile: Avoid $(objpfx)/ in makefiles

[Florian Weimer]

* Florian Weimer:

> If paths with both $(objpfx)/ and $(objpfx) (which already includes
> a trailing slash) appear during the build, this can trigger unexpected
> rebuilds, or incorrect concurrent rebuilds.
>
> Tested on x86_64-linux-gnu.
>
> ---
>  elf/Makefile | 12 ++++++------
>  nss/Makefile | 32 ++++++++++++++++----------------
>  2 files changed, 22 insertions(+), 22 deletions(-)

Our testing shows that this indeed fixes the spurious rebuild issue we
encountered before.

Thanks,
Florian