John's revert of my removal commit dragged back a bunch of cygwin/sunos
cruft, as well as re-adding RegExp.cpp to the host libshared, that we don't
need.
Instead, remove this and add libgnuregex_build to just the tools/keymap
link alongside the FreeBSD gnuregex case.
It was needed on macOS for a time when BUrl used regexes for parsing.
Now it does not, and so we can remove libshared's RegExp from build
libshared, and thus also libgnuregex.
The previous commit only checked that they worked if the user
enabled them, but now they will be enabled automatically if the
underlying filesystem supports them.
The ReadMe.Compiling has been updated accordingly.
