add a directive to disable SwitchTransform

Previously cython.inline didn't correctly handle variables
declared in nested scopes (such as function declarations).

SwitchTransform is unable to detect all cases of duplicate values, which
result in errors at compile time in the generated code.

utility code construction for struct conversion requires constructing the field conversion functions as well

--HG--
extra : amend_source : af016229b6c8c40168070e4d3a9581ed12500d3f

utility code construction for struct conversion requires constructing the field conversion functions as well

--HG--
extra : transplant_source : %A3%81%87%E2%0A%20%DA%7D%7E%5B%A0%9B%80i%F9%8Az%9B%F3%DC

Compute each Makefile variable immediately

Otherwise the variables get re-computed over and over again every time
they are used, slowing things up.  Especially if someone adapts the
Makefile to several targets in their own real project. :)

revert to using codecs.open() for output files instead of io.open() as the latter cannot write str objects in text mode under Py2 (and we erroneously do that in some spots)

make encoding detection code a bit faster if the first two source file lines are longer than 250 bytes