This is a first time install for this package as it is now pulled on a Gnome desktop via gnome-photos and gegl. It fails to build looking for jah_config.h which is multilib specific directory and not in src.
append-cppflags "-I${BUILD_DIR}/src/libjasper/include" should do
Please post a bug report or an error or something. I cannot reproduce this problem.
Found the bug, should be fixed now. commit 748ff9670d5144df8e87a49e7031740b3d77214c Author: David Seifert <soap@gentoo.org> Date: Tue Oct 18 20:23:25 2016 +0200 media-libs/jasper: Version bump to 1.900.6 Gentoo-bug: 597208