Merge pull request #2618 from andrjohns/feature/boost-lgamma

Allow forcing use of Boost lgamma on Unix

Author: rok-cesnovar

stan/math/prim/fun/lgamma.hpp

@@ -12,7 +12,7 @@
  * back. For details on the speed evaluations, please refer to
  * https://github.com/stan-dev/math/pull/1255 .
  */
-#if !__MINGW32__
+#if !__MINGW32__ && !_BOOST_LGAMMA
 // _REENTRANT must be defined during compilation to ensure that cmath
 // exports the reentrant safe lgamma_r version.
 #if !_REENTRANT
@@ -61,7 +61,7 @@ namespace math {
 * argument
 */
 inline double lgamma(double x) {
-#if !__MINGW32__
+#if !__MINGW32__ && !_BOOST_LGAMMA
   int sign = 1;
   return ::lgamma_r(x, &sign);
 #else
@@ -80,7 +80,7 @@ inline double lgamma(double x) {
  * argument
  */
 inline double lgamma(int x) {
-#if !__MINGW32__
+#if !__MINGW32__ && !_BOOST_LGAMMA
   int sign = 1;
   return ::lgamma_r(x, &sign);
 #else