Allow constraint tolerance to be adjusted with a compile-time define

Author: WardBrian

stan/math/prim/err/constraint_tolerance.hpp

@@ -1,15 +1,21 @@
 #ifndef STAN_MATH_PRIM_ERR_CONSTRAINT_TOLERANCE_HPP
 #define STAN_MATH_PRIM_ERR_CONSTRAINT_TOLERANCE_HPP
 
+#ifndef STAN_MATH_CONSTRAINT_TOLERANCE
+#define STAN_MATH_CONSTRAINT_TOLERANCE 1E-8
+#endif
+
 #include <stan/math/prim/meta.hpp>
 namespace stan {
 namespace math {
 
 /**
  * The tolerance for checking arithmetic bounds in rank and in
  * simplexes.  The default value is <code>1E-8</code>.
+ * This can changed by defining <code>STAN_MATH_CONSTRAINT_TOLERANCE</code>
+ * at compile time.
  */
-const double CONSTRAINT_TOLERANCE = 1E-8;
+const double CONSTRAINT_TOLERANCE = STAN_MATH_CONSTRAINT_TOLERANCE;
 
 }  // namespace math
 }  // namespace stan