Upstream states that luajit built with Lua 5.2 support is required.
See http://devel.aegisub.org/ticket/1904#comment:2
Package-Manager: portage-2.2.26
# However, most of these minimal versions date back to 2006-2010 yy.
# Such version specifiers are meaningless nowadays, so they are omitted.
RDEPEND="
- >=dev-lang/luajit-2.0.4:2
+ >=dev-lang/luajit-2.0.4:2[lua52compat]
>=dev-libs/boost-1.50.0:=[icu,nls,threads]
>=dev-libs/icu-4.8.1.1:=
>=x11-libs/wxGTK-3.0.0:${WX_GTK_VER}[X,opengl,debug?]