net-libs/webkit-gtk: bump to 2.18.1 for security; other package touchups

net-libs/webkit-gtk: bump to 2.18.1 for security; other package touchups

- reorder deps to cmake checks order
- add missing zlib dep
- move gtk platform USE checks to the main depstring, as that's requiring newer version anyway
- remove unnecessary pango dep (not checked in cmake, minimal usage comes via gtk+ deps)
- add WebCrypto deps, without giving optionality (it's a private option)
- add missing cmake-utils_src_prepare call
- remove ancient <glibc-2.18 code
- fix jsc pax-mark
- remove ruby21 support (gone from tree and repoman warning about the slot)
- GL deps and conditions remain a mystery to me for now

Closes: https://bugs.gentoo.org/626736
Package-Manager: Portage-2.3.8, Repoman-2.3.2