CURL_SSL is not needed. Other packages support different tls/ssl implementations simply with USE flags. It can be simply done here as well.
Examples can be found here: https://wiki.gentoo.org/wiki/Project:LibreSSL
!libressl? ( dev-libs/openssl:0= )
libressl? ( dev-libs/libressl:0= )
polarssl? ( net-libs/mbedtls:= )
gnutls? ( net-libs/gnutls:= )
nettle? ( dev-libs/nettle:= )
NB: "The important point to note is that USE=libressl acts as a discriminating flag between OpenSSL and LibreSSL. As such, it is not a pure implementation USE flag and is analogous to USE=libav. That is, USE=libressl is never mutually exclusive to any other USE flag and should therefore never appear in REQUIRED_USE".
can also be replaced with
(axtls and cyassl are no longer relevant, AFAIK)
I think this will also fix #588060 - cf. https://bugs.gentoo.org/588060#c4