Doc: improve directions for building on macOS.

Doc: improve directions for building on macOS.

In light of recent discussions, we should instruct people to
install Apple's command line tools; installing Xcode is secondary.

Also, fix sample command for finding out the default sysroot,
as we now know that the command originally recommended can give
a result that doesn't match your OS version.

Also document the workaround to use if you really don't want
configure to select a sysroot at all.

Discussion: https://postgr.es/m/20210119111625[email protected]