hi. the boehm-gc-6.2 ebuild only installs ${S}/include/*.h into /usr/include/gc but not ${S}/include/private/*.h, which breaks every application depending on it. fix is to add these 2 lines to the ebuild: insinto /usr/include/gc/private doins include/private/*.h right under the installation of the other headers.
yup, at least AFAIK old w3m needs gc/private headers. Fixed in boehm-gc-0.6.2-r1.ebuild. Thanks for reporting.