Merge pull request #918 from mneumann/dev-fix-build-on-dragonflybsd

Fix build on FreeBSD-derivate DragonFly

Author: daanx

src/alloc-override.c

@@ -248,7 +248,7 @@ extern "C" {
   // Forward Posix/Unix calls as well
   void*  reallocf(void* p, size_t newsize) MI_FORWARD2(mi_reallocf,p,newsize)
   size_t malloc_size(const void* p)        MI_FORWARD1(mi_usable_size,p)
-  #if !defined(__ANDROID__) && !defined(__FreeBSD__)
+  #if !defined(__ANDROID__) && !defined(__FreeBSD__) && !defined(__DragonFly__)
   size_t malloc_usable_size(void *p)       MI_FORWARD1(mi_usable_size,p)
   #else
   size_t malloc_usable_size(const void *p) MI_FORWARD1(mi_usable_size,p)