Merge pull request #3268 from stan-dev/minor-fix

minor change to use constant

Author: WardBrian

stan/math/prim/fun/log1m_exp.hpp

@@ -49,7 +49,7 @@ inline double log1m_exp(double a) {
   using std::log;
   if (a > 0) {
     return NOT_A_NUMBER;
-  } else if (a > -0.693147) {
+  } else if (a > LOG_HALF) {
     return log(-expm1(a));  // 0.693147 ~= log(2)
   } else {
     return log1m(exp(a));