This makes our builds a bit more reproducible. Without it when processing
#include "IPC/IPCMessageUtils.h
we end up in
#include "ShadowLayers.h"
which in the final XUL translates into the symbol (run nm -a to see it):
- 00 0000 SOL ../../../../gfx/layers/IPC/ShadowLayers.h
now when some file in gfx/layers/ipc does
#include "ShadowLayres.h"
the file found is the local one with the lowercase directory name and we end up
with
- 00 0000 SOL ../../../../gfx/layers/ipc/ShadowLayers.h
