I'm not convinced that would help much.  The GNULD variable in the
makefile is for when the default linker is used.  If you change that by
using LDSHARED then you're probably not going to be using --rpath but
LDFLAGS to configure it the way you want.

If anything maybe using configure/Makefile to detect if GNU ld is used
is wrong just for the case they use LDSHARED (I didn't think of this
before), since then they can use LDSHARED and --rpath and get misterious
failures.  But it seems a lot more complicated to do, LDSHARED can be
set to something like "cc -shared" (the default) in which case we can't
use -V and assume it's a non-GNU ld if we don't get "GNU" back.  So we'd
have to try and detect if LDSHARED is set to a compiler or a linker,
then try to find which linker gets invoked etc.  A lot more complicated
and way more possibilities then I can test.

I'd argue that when someone uses LDSHARED they should be using LDFLAGS
intead of --rpath, they obviously know what they are doing.  --rpath is
there if you want to use the environment Python was compiled in to build
an extension module with a RPATH/RUNPATH in.