Document macOS commands to install dependencies

[rburing]

This is not really essential, but would be nice to have so not everyone has to figure this out by themselves.

The README includes the Ubuntu command

apt install libgmp-dev libmpfr-dev make autoconf libtool-bin

I propose copying this to the beginning of https://flintlib.org/doc/building.html, and adding e.g.

brew install gmp mpfr autoconf libtool

for macOS, along with a link to https://brew.sh/ there.

This is not yet the complete setup, because I also got make and a compiler somewhere (and not by brew install make). Perhaps it was xcode-select –-install.

When using brew I also had to add

--with-gmp-include=/opt/homebrew/include --with-gmp-lib=/opt/homebrew/lib --with-mpfr-include=/opt/homebrew/include --with-mpfr-lib=/opt/homebrew/lib

to the ./configure flags for FLINT. I am no macOS expert, hence this issue and not yet a PR.