www-client/firefox: adjust requirements

www-client/firefox: adjust requirements

- Warn user when USE=lto is set but used compiler is outdated.
  [Bug #666784]

- Enforce >=rust-1.28.0. [Bug #669394]

- Disable ELF hack again. [Bug #669382]

- Enforce usage of >=dev-libs/nss-3.39. [Bug #669374]

Bug: https://bugs.gentoo.org/666784
Closes: https://bugs.gentoo.org/669394
Closes: https://bugs.gentoo.org/669382
Closes: https://bugs.gentoo.org/669374
Package-Manager: Portage-2.3.51, Repoman-2.3.11
Signed-off-by: Thomas Deutschmann <whissi@gentoo.org>