1) add pcre dependency (for some reason builtin_pcre doesn't work)
2) Disable dependencies that are currently not supported by the
expression. Most users should not need those. These are disabled to
prevent cmake from picking them up from system and causing impurities.
Once there is a user who needs these they will have to update the
expression.
3) disable some OSX detection code that relies on /usr/bin/sw_vers
that chooses c++ library, silences warnings and sets macosx-version-min.
macosx-version-min is already set by nix using MACOSX_DEPLOYMENT_TARGET
environment variable.
