No real changes, just remove an unused variable.

base_abs was never used in Formatter.path().
Fix paths for the dependencies in the generated makefiles.

The changes of 58ffebfe95b3e64b1b43c639cf8fda4302d25e16 broke the paths for
the dependencies in different directories as they simplified a bit too much.
Restore the non $(builddir)-related part of MakefileToolset.get_builddir_for
to correct this and fix the discrepancy between targets referenced outside of
their own makefile and those only referenced in it by using special new
ANCHOR_TOP_BUIDLDIR for the former.
Fix clean commands when using build directory with gnu backend.

The files with auto-cleaned extensions are generated in the build directory,
not the source one, so remove them from there.