-DWITH_SYSTEM_GLFW=ON: Link against system glfw and fail otherwise
-DWITH_SYSTEM_GLFW=OFF: Use embedded rglfw.c
-DWITH_SYSTEM_GLFW=IF_POSSIBLE: Probe for system glfw but fallback to
rglfw if unavailable
Also change Linux 64-bit CI build to install system glfw and use it,
so this doesn't bitrot.
Addresses #453.