pass through enable malloc build for dragonflybsd. (#614)

CMakeLists.txt

@@ -452,7 +452,6 @@ if(NOT SNMALLOC_HEADER_ONLY_LIBRARY)
     if (WIN32
     OR (CMAKE_SYSTEM_NAME STREQUAL NetBSD)
     OR (CMAKE_SYSTEM_NAME STREQUAL OpenBSD)
-    OR (CMAKE_SYSTEM_NAME STREQUAL DragonFly)
     OR (CMAKE_SYSTEM_NAME STREQUAL SunOS))
     # Windows does not support aligned allocation well enough
     # for pass through.

src/snmalloc/mem/external_alloc.h

@@ -45,6 +45,11 @@ namespace snmalloc::external_alloc
 {
   using ::malloc_usable_size;
 }
+#    elif defined(__DragonFly__)
+namespace snmalloc::external_alloc
+{
+  using ::malloc_usable_size;
+}
 #    else
 #      error Define malloc size macro for this platform.
 #    endif