make fpos_t an opaque type

some missing math.h functions and macros in math.pxd

use -fPIC -shared when compiling "libraries" demo

--HG--
extra : rebase_source : 7b763fbb0f2485056ec4d2f53a5ec76dc7be9203

This fixes the "libraries" demo's setup.py for building the call_mymath
extension.

Fix grammar/typo in build docs

There was a word missing.

Avoid dramatic changes in the resulting .c file when the input .pyx file changes only slightly.