Set ROOT to top_builddir

[proski]

Building with MSYS2 was failing due to the command line for linking glib DLL exceeding the sh.exe limit of 8192 bytes. When sh.exe from MSYS2 was present in PATH, gmake would use it and run a truncated command.

Using relative filenames shortens that command line to about 5k regardless of the source directory location.

[proski]

This PR is for topic/build-228, I understand that';s the current "development branch".
I'm not a fan of ?= for variables essential for the build. It's more appropriate for VERBOSE and other superficial stuff.
I'm not sure about the impact on other projectd, maybe $(top_builddir) should be used instead of $(ROOT) in the makefiles to avoid unnecessary indirection.

[proski]

By the way, it should be possible to make win32 builds more similar to the autotools builds.
Current directory structure:

root = top_builddir = top_scrdir
    bin.compiler
        release
            binaries
    lib.compiler
        release
            libraries
    obj.compiler
        release
            object files

Better structure:

root = top_scrdir
    release.compiler = top_builddir
        bin
            binaries
        lib
            libraries
        obj
            object files

As the next step, the generated makefiles could be created under top_builddir, allowing multiple builds from the same source to be run in parallel.

[Neustradamus]

@adamyg: Why this PR has been closed?