override BIN_DIR in makefile for gentoo install

This change seems to be all that is necessary for a basic, working
ebuild; eventually I want to support doc, test, and linguas USE flags,
all of which will require patching the makefile for the ebuild, and to
fix and/or investigate what portage thinks is a syntax error in the
.desktop file:

/usr/share/applications/edgar.desktop: error: value "Game" for string
list key "Categories" in group "Desktop Entry" does not have a semicolon
(';') as trailing character