Add MSVC-specific #define to expose math constants in <cmath>.

- MSVC does not expose standard math constants, such as M_PI in <cmath>
  or <math.h> unless _USE_MATH_DEFINES is defined prior to their
  inclusion: https://msdn.microsoft.com/en-us/library/4hwaceh6.aspx.
- Use CMake to ensure that this is #defined when both Ceres and the
  examples are compiled, even though it should only be an issue for the
  examples where M_PI is used.

Change-Id: I67af75b100b8138a65514273d23bfe445d92652c