Cleanup: Fix build error with MSVC

Previously eigens internal include order somehow implicitly provided
M_PI and friends. The recent eigen version bump broke this implicit
behaviour, better to be explicit that we need the math defines for MSVC.

intern/iksolver/CMakeLists.txt

@@ -27,6 +27,10 @@ set(INC_SYS
   ${EIGEN3_INCLUDE_DIRS}
 )
 
+if(WIN32)
+  add_definitions(-D_USE_MATH_DEFINES)
+endif()
+
 set(SRC
   intern/IK_QJacobian.cpp
   intern/IK_QJacobianSolver.cpp