Enable building of math based libraries on win32 system

The concerned libraries are: cmath, datetime and time

On windows, math functions are provided by "C runtime" and no specific
library is required.
See http://msdn.microsoft.com/en-us/library/x39715t6(v=vs.71).aspx
and http://msdn.microsoft.com/en-us/library/abx4dbyh(v=vs.71).aspx

For that reason, the variable M_LIBRARIES has been introduced and is
set to the value of HAVE_LIBM on UNIX system and set to an emptry
string on win32 system.
Add msvcrt module

Since the source of the module doesn't live under the Modules folder,
the extension macro has been updated to support absolute dir.

Note that more work is required to support absolute dir for assembler file.
Add subprocess win32 module
Add _winreg win32 module
Add _msi win32 module
On windows, install headers in expected sub folder.

As specified in the "get_python_inc" function provided by the sysconfig
module, headers are expected to be inside the "include" directory.
See http://hg.python.org/cpython/file/6fc9103d55f0/Lib/distutils/sysconfig.py#l95

Note that for now only the (1) in-source build tree and (2) install tree
are compliant.
Merge branch 'add-missing-win32-modules'

* add-missing-win32-modules:
  On windows, install headers in expected sub folder.
  Add _msi win32 module
  Add _winreg win32 module
  Add subprocess win32 module
  Add msvcrt module
  Enable building of math based libraries on win32 system