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 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler