bug#58080: 29.0.50; Don't hard-code path to pwd

[Jonas Bernoulli]

On Guix pwd isn't located at /bin/pwd, but that path is hard-coded in
various Makefile.in.  They deal with that by patching these files when
building the emacs package, but when building emacs manually, that isn't
convenient, as doing so would leave the working tree dirty, which makes
it harder to pull whenever one of these files changes.

Please consider making this customizable so one could use, for example:

  make PWDCMD=pwd install

I assume there is a reason why the PWD variable isn't being used and
that /bin/pwd is used because on some other OS using pwd would cause
issues.