Automatically search for GMP and GNU Readline in a few additional places, if we cannot find them elsewhere

[fingolfin]

When searching for GMP and GNU Readline, if we fail to detect them w/o specific compiler flags, then we could iterate over a list of standard locations where they might be found instead. Specifically, for OS X, that could be the include and lib subdirectories of...

• /sw, for a typical Fink installation;
• /opt/local, for a typical MacPorts install;
• $(brew --prefix) for Homebrew (and for GNU Readline, also $(brew --prefix)/opt/readline as it is installed keg-only by default)

I am not sure if any other operating system resp. distros would benefit from it, but I'd think it'd be worth the effort regardless of that.

[fingolfin]

With macOS 11 "Big Sur", this is getting more important, as there, Homebrew is installed into /opt/homebrew, and so GAP won't find its GMP or readline unless the user specifies flags.

[fingolfin]

For now, this is what I did on an M1 Mac:

export HOMEBREW_PREFIX=$(brew --prefix)
./configure --with-gmp=$HOMEBREW_PREFIX --with-readline=$HOMEBREW_PREFIX/opt/readline

[fingolfin]

Alternatively, we could also offer a --with-homebrew[=PREFIX] option (if not prefix is given, it would try to look for a brew command and use brew --prefix).

This could then also alter the default for --with-readline so that it looks in the right dir.

But of course overall it'd still be nicest if users could just do ./configure && make and it would "just work" by default -- but I'd be careful to preserve the ability to override / disable any such defaults, because the user may explicitly want to build GAP against a different readline/GMP/zlib, or build without readline support, etc. -- e.g. for testing purposes. I'll leave open how this override works (e.g. --without-readline or --with-gmp=builtin or --with-zlib=ALTERNATE-ZLIB-PATH etc. could do it -- that's how it's done right now, I think).

This is not hard to ensure, as long as one is aware of the need to do so, which is why I mention it here explicitly :-).

[fingolfin]

Oh and note that autoconf already has features which take a list of possible prefixes and search for tools / headers / libraries in there. So a standard thing configure scripts do is to have a list of locations to search, and augment that depending on the target system: E.g. searchpaths=/usr /usr/local and then append /opt/homebrew or /sw (Fink) or /opt/local (MacPorts) or $(brew --prefix) (Homebrew, Linuxbrew?, ...) etc. as needed -- some configure scripts even have an option like --with-searchpath=PREFIX that allows the user to specify additional such search paths.

This can be super convenient -- however, it can also be super frustrating to some users, and esp. to packagers, if it cannot be turned off / overridden.