When you invoke programs under WSL, WSL does not do automatic path name translation for filename arguments like msys does. So providing Windows-style paths (C:\...) in WSL isn't going to work very well. Fixing this (i.e. providing relative paths rather than absolute paths) for mt.exe is relatively straightforward. Ted wrote these patches, and I rebased them around some changes. I'm not completely sure they're the best way to handle things, so I thought I would get a second opinion. If we go this route, there are undoubtedly more places to clean up.

Many Windows tools don't deal well with absolute paths for filenames, oddly enough, so we want to pass relative paths into the source directory where possible. These relative paths also mean that they look like "normal" unix paths to WSL and therefore don't require any special handling.

mt.exe doesn't like absolute paths, so we use srcdir-relative paths where necessary for file arguments.

BUILD_TOOLS was only ever used for things that another variable provides equally well. Removing BUILD_TOOLS means that we can remove win_srcdir and WIN_TOP_SRC as well.

Pushed by nfroyd@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/263fb3ff055c part 1 - emit a srcdir_rel variable in every directory; r=mshal https://hg.mozilla.org/integration/mozilla-inbound/rev/23cb01b0c33e part 2 - use srcdir_rel for passing manifest paths to mt.exe; r=mshal https://hg.mozilla.org/integration/mozilla-inbound/rev/b3782814d458 part 3 - remove BUILD_TOOLS from config.mk; r=mshal

https://hg.mozilla.org/mozilla-central/rev/263fb3ff055c https://hg.mozilla.org/mozilla-central/rev/23cb01b0c33e https://hg.mozilla.org/mozilla-central/rev/b3782814d458