build: simplify gnu-getopt search

build: simplify gnu-getopt search

getopt is the only command where /usr/local/bin is specified explicitly.
All other commands are assumed to exist in the PATH in one form or
another.  Remove this exception and require gnugetopt/getopt to be in
the user's PATH.

In the case of macos Homebrew, getopt is 'keg only' hence not linked
into /usr/local/bin whilst other commands are linked and likely found by
virtue of /usr/local/bin being in PATH.

Since 2019 Homebrew is very reluctant to install links that have
potential to override default OS behaviour, eg: following instructions
on our current 'how to build on macos' wiki page:

$ brew ln gnu-getopt --force
Warning: Refusing to link macOS-provided software: gnu-getopt
If you need to have gnu-getopt first in your PATH run:
  echo 'export PATH="/usr/local/opt/gnu-getopt/bin:$PATH"' >> ~/.zshrc

A better option for macos is to link getopt as 'gnugetopt' in
/usr/local/bin, thus the build system will find 'gnugetopt' but other
applications looking for just 'getopt' will find the original macos
binary.

Ultimately it makes sense that 'GNU' dependencies are placed in
/usr/local/bin and /usr/local/bin is included in the user's PATH.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>