db0912c6b2
Introduce "--with-system-gbm" option, allowing to build Firefox against system gbm library and to find system gbm header files. Also unify all gbm includes to match both system and bundled paths. Differential Revision: https://phabricator.services.mozilla.com/D242098
Libgbm is a gbm library wrapper needed to build and run Firefox with Pipewire support on Linux (https://gitlab.freedesktop.org/mesa/gbm).
libgbm directory stores headers of libgbm needed for build only. gbm stores the vendored upstream header.