add an implementation for max for long (ssize_t are long typedef'ed).
Otherwise it will fail in parallel_partition.h:185 due to an ambiguous
call.  Backport of https://github.com/embree/embree/pull/379

Index: thirdparty/embree/common/math/math.h
--- thirdparty/embree/common/math/math.h.orig
+++ thirdparty/embree/common/math/math.h
@@ -239,6 +239,7 @@ namespace embree
     __forceinline T twice(const T& a) { return a+a; }
 
   __forceinline      int min(int      a, int      b) { return a<b ? a:b; }
+  __forceinline     long min(long     a, long     b) { return a<b ? a:b; }
   __forceinline unsigned min(unsigned a, unsigned b) { return a<b ? a:b; }
   __forceinline  int64_t min(int64_t  a, int64_t  b) { return a<b ? a:b; }
   __forceinline    float min(float    a, float    b) { return a<b ? a:b; }
@@ -259,6 +260,7 @@ namespace embree
   template<typename T> __forceinline T mini(const T& a, const T& b, const T& c, const T& d, const T& e) { return mini(mini(mini(a,b),mini(c,d)),e); }
 
   __forceinline      int max(int      a, int      b) { return a<b ? b:a; }
+  __forceinline     long max(long     a, long     b) { return a<b ? b:a; }
   __forceinline unsigned max(unsigned a, unsigned b) { return a<b ? b:a; }
   __forceinline  int64_t max(int64_t  a, int64_t  b) { return a<b ? b:a; }
   __forceinline    float max(float    a, float    b) { return a<b ? b:a; }
