net-ftp/gproftpd: Fix building with CFLAGS=-fno-common

net-ftp/gproftpd: Fix building with CFLAGS=-fno-common

Also:
- EAPI=7
- Use HTTPS (except for SRC_URI which was already failing)
- Drop pointless USE flags (see below)
- Fix dependencies: openssl is not used (drop USE=ssl), proftpd should not be a
  dependency at all as gproftpd merely edits its run-time configuration
  files, add gettext/libiconv
- Fix .desktop file and install it at the correct location (drop
  USE=gnome)
- Drop configuration flags that very obviously belong to proftpd and not
  gproftpd

Package-Manager: Portage-2.3.100, Repoman-2.3.22
Closes: https://bugs.gentoo.org/710878
Signed-off-by: Jeroen Roovers <jer@gentoo.org>