missing --with-pkgconfigdir (PKG_INSTALLDIR) support in pkgconf's copy of pkg.m4, it will need to be updated from the freedesktop.org's copy
consider doing something like what dev-util/pkgconfig-openbsd is already doing, copying the pkg.m4 from the freedesktop.org's tarball, so it would be a matter of changing one string in the ebuild, like PKG_M4_VER=0.26 to PKG_M4_VER=0.27
upstream rebase patch: https://github.com/nenolod/pkgconf/commit/840ecc846d1a168b475a31bb927b78c3677d565a.patch
The patch is being applied to pkgconf-0.8.1 in the tree.