As described in the upstream bugreport, gtk-doc.make looks for generated docs in $(srcdir) instead of $(builddir). This makes ebuilds using out-of-source builds unable to install docs. I'm opening a Gentoo bug to keep track of this issue and to know when my ebuild will start working as expected.
+*gtk-doc-1.17-r1 (10 Sep 2011) + + 10 Sep 2011; Pacho Ramos <pacho@gentoo.org> + -files/gtk-doc-1.13-fixxref-vim-u-NONE.patch, -gtk-doc-1.15.ebuild, + -gtk-doc-1.15-r2.ebuild, + -files/gtk-doc-1.15-allow-selection-highlighter.patch, + -files/gtk-doc-1.15-fixxref-vim-fixes.patch, +gtk-doc-1.17-r1.ebuild, + +files/gtk-doc-1.17-buildir-docs.patch: + When doing 'make install', look for generated docs in builddir, bug #379415 + by Michał Górny. Remove old. +