I have found that ebuilds for gtkmm and glibmm don't specify explicitly whether to build static library or not. As a result, static version is built on x86 and not built on amd64. Since I need static version of these libraries (I link my program with them when I'm going to send binary to someone, since c++ abi changes between gcc versions, and these libraries is less common than gtk and glib), I have to patch the ebuilds on amd64 system to get them build.
I suggest adding 'static' use-flag to these ebuilds (and, perhaps, to some others), or even to more generic eclass (such as gnome2). This will make possible to control whether to build static libraries or not.
No it shouldn't. File bugs about ebuilds that don't install both static and shared libs if it's possible for them to compile and install both. USE=static is NOT intended for this but only to make a binary use static linking instead of dynamic linking, cf. Bug 165629 and Bug 165361.
Bug 74801 reopened for your -mm stuff issues.