Fix CFLAGS environ in tox config

Add signbit to math.pxd.

Also reorder a few imports to alphabetical order.

Conflicts:
	Cython/Shadow.py

keep C++ code out of C files and vice-versa by splitting module preamble (original patch by Lars Mans)

exclude two more modules from stdlib compilation that require bootstrapping before importing Cython modules

Write "generated by Cython" header also in .h files

exclude two more modules from stdlib compilation that require bootstrapping before importing Cython modules

Fix the placement of a closing bracket