Create fake pacman packages to satisfy dependencies that should be optdepends in the first place!
This script generates a PKGBUILD for an almost empty package, then builds and installs it with makepkg. Run ./makefake --help for usage, or just run ./makefake --version 1.2.3 legitpackage for the most common scenario. Your fake package can provide multiple packages, just add more package names.
Note that IgnorePkg in /etc/pacman.conf achieves similar results - this script should be used only if that solution doesn't satisfy you for some reason. In my case that's because I sync /etc/pacman.conf between all my computers and cannot install certain packages on one of them due to odd constraints on that machine. Just faking some packages is easier this way.
DON'T REPORT BUGS IN ARCH PACKAGES IF YOU HAVE FAKED ANY DEPENDENCIES IN THEIR PACTREE. DON'T FAKE BASE OR BASE-DEVEL PACKAGES. If you want to fake install gtk3, sure, you can do that, but don't be surprised when all Gtk+3 apps stop working. This is a dangerous tool - use it wisely and sparingly.