math.h change: add the _USE_MATH_DEFINES flag

By Patrick Spendrin <ps_ml@gmx.de>
  according to the msdn documentation
  http://msdn.microsoft.com/en-us/library/4hwaceh6.aspx _USE_MATH_DEFINES
  should give constants like M_PI etc.

  This patch adds it.

  regards,
  Patrick

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/trunk@6617 4407c894-4637-0410-b4f5-ada5f102cad1
(cherry picked from commit 47a6c7830dc57db1e11d850ff3861857e6b59466)