Update Windows build to MinGW-w64 4.9.1

Since different versions of MinGW-w64 differ in support of clock-related
functions, we do now suggest using version 4.9.1 which was tested on
several Windows systems.

This fix removes the now obsolete redefinitions of unsupported clock
features and gives detailed instructions in the README for fetching the
correct version of MinGW for Windows builds.
Cut down on automatic header inclusion on Windows

This will prevent windows.h from dragging in lots of other header files
by default. It helps us avoid problems such as having to include
winsock2.h before windows.h which would otherwise lead to problems with
redefined symbols.
Remove -fPIC for builds on Windows

We recommend using MinGW-w64 for the Windows builds which generates
position-independent code by default. Removing the -fPIC option gets
rid of the warning that reminds of this behaviour.
Fix weird numbering in Windows README file
Remove hh modifier from format strings

MinGW does not support C99's hh modifier in format strings (likely for
compatibility with Microsoft's compiler). This fix replaces the modifier
with a cast of char arguments to unsigned char to achieve the same result.
Merge pull request #136 from nitsch/fix-windows-build

Fix Windows build