Add `gmp` as an addition dependency for macOS

[postmodern]

Add gmp as an additional dependency for macOS to accelerate the Bignum class. Will need to determine the gmp package name for brew and MacPorts.

[havenwood]

It looks like the package name is gmp for both Homebrew and MacPorts.

• https://formulae.brew.sh/formula/gmp
• https://ports.macports.org/port/gmp

[postmodern]

We probably should add the gmp package for all of the other package managers as well, especially pkg (FreeBSD).