This also fixes breakage on darwin due to LLVM4.0 migration.
I had to enable opengl because otherwise macOS build is broken.
See root commit 23f8871c7 ('Do not include OpenGL directly but use our
TGLIncludes for that purpose.')
I've enabled xml because TUnfold depends on it.
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.
