The build didn't work for me at first.
Workaround:
ln -s /usr/lib64/pkgconfig/slv2.pc /usr/lib64/pkgconfig/libslv2.pc
emerge ingen
stop emerge ingen after src unpack
ln -s /usr/lib64 \
/var/tmp/portage/media-sound/ingen-9999/work/ingen-9999/lv2/lv2/osc/.libs
ln -s /usr/lib64 \
/var/tmp/portage/media-sound/ingen-9999/work/ingen-9999/redlandmm/src/.libs
restart emerge