auxiliary: rework the python generated sources rules

auxiliary: rework the python generated sources rules

There are a few bits this commit aims to resolve:

One can generalise the mkdir rule to a simple MKDIR_P $(@D) which will
expand appropriately for even if we change the subdir name, and/or add
new rules. We can also drop the explicit $(srcdir) prefix for the
dependency rules, they they are not strictly required, nor used
elsewhere in mesa.

Finally replace $< with explicit filename to be consistent through the
file, and honour PYTHON_FLAGS.

v2: Add comprehensive commit summary/message (Ian, Matt)

Cc: 11.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>