Use _j[0,1,n]() Bessel functions on MSVC to avoid deprecation errors.

- Microsoft deprecated the POSIX Bessel functions: j[0,1,n]() in favour
  of _j[0,1,n](), it appears since at least MSVC 2005:
  https://msdn.microsoft.com/en-us/library/ms235384(v=vs.100).aspx.
- As this occurs in jet.h (templated public header), although Ceres
  suppresses the warning when it itself is built (to suppress a warning
  about the insecurity of using std::copy), it will crop up again in
  client code (without this fix) unless it is explicitly suppressed
  there also.
- Raised as Issue #190:
  https://github.com/ceres-solver/ceres-solver/issues/190.

Change-Id: If7ac5dbb856748f9900be93ec0452a40c0b00524

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index b89f807..fc98d44 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -506,9 +506,32 @@
 }
 
 // Bessel functions of the first kind with integer order equal to 0, 1, n.
-inline double BesselJ0(double x) { return j0(x); }
-inline double BesselJ1(double x) { return j1(x); }
-inline double BesselJn(int n, double x) { return jn(n, x); }
+//
+// Microsoft has deprecated the j[0,1,n]() POSIX Bessel functions in favour of
+// _j[0,1,n]().  Where available on MSVC, use _j[0,1,n]() to avoid deprecated
+// function errors in client code (the specific warning is suppressed when
+// Ceres itself is built).
+inline double BesselJ0(double x) {
+#if defined(_MSC_VER) && defined(_j0)
+  return _j0(x);
+#else
+  return j0(x);
+#endif
+}
+inline double BesselJ1(double x) {
+#if defined(_MSC_VER) && defined(_j1)
+  return _j1(x);
+#else
+  return j1(x);
+#endif
+}
+inline double BesselJn(int n, double x) {
+#if defined(_MSC_VER) && defined(_jn)
+  return _jn(n, x);
+#else
+  return jn(n, x);
+#endif
+}
 
 // For the formulae of the derivatives of the Bessel functions see the book:
 // Olver, Lozier, Boisvert, Clark, NIST Handbook of Mathematical Functions,