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.
Merge branch 'enable-math-based-libraries-on-win32'

* enable-math-based-libraries-on-win32:
  Enable building of math based libraries on win32 system