app-text/scdoc: fix cross-compile, prefix install.

app-text/scdoc: fix cross-compile, prefix install.

Cross-compiling (tested with a crossdev musl toolchain via
`PORTAGE_CONFIGROOT=/usr/x86_64-pc-linux-musl ebuild scdoc-1.3.1.ebuild ...`
currently fails because VERSION is not defined.

Installing in prefix fails due to files being installed outside of the
prefix. Pass PREFIX="${EPREFIX}/usr" on emake call.

Tested via building with `ebuild scdoc-1.3.1.ebuild ...` while inside a
prefix.

Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>
Package-Manager: Portage-2.3.51, Repoman-2.3.11
Signed-off-by: Michał Górny <mgorny@gentoo.org>