dev-lang/erlang: version bump to 21.1

dev-lang/erlang: version bump to 21.1

I removed some deprecated configure options, properly (re-)added the
"wxwidgets" USE flag - without automagic dependencies, added the "pgo"
flag and made some USE flags enabled by default to match upstream's
defaults.

Most of the ebuild was refactored, after code reviews, with the more
significant change being the replacement of two shell functions copied
from an upstream build script with a call to a patched version of that
script.

When the "pgo" USE flag is enabled, distcc and ccache are disabled, to
avoid a build failure due to a version mismatch in .gcda files that
cannot be explained by different GCC versions.

Closes: https://bugs.gentoo.org/653700
Closes: https://bugs.gentoo.org/658656
Signed-off-by: Ștefan Talpalaru <stefantalpalaru@yahoo.com>
Signed-off-by: Michał Górny <mgorny@gentoo.org>
Closes: https://github.com/gentoo/gentoo/pull/9357