core: fix a #if vs #ifdef for constructor attribute support

Unnoticed until now because these lines were added recently,
way after all compilers added support for this attribute...
until somebody tried autoconf with MSVC.

Thanks to Alexander Neumann for the report.

Closes #485

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

hwloc/topology.c

@@ -69,7 +69,7 @@
  * it will break in cygwin, we'll have to use both putenv() and SetEnvironmentVariable().
  * Hopefully L0 will be provide a way to enable Sysman without env vars before it happens.
  */
-#ifdef HWLOC_HAVE_ATTRIBUTE_CONSTRUCTOR
+#if HWLOC_HAVE_ATTRIBUTE_CONSTRUCTOR
 static void hwloc_constructor(void) __attribute__((constructor));
 static void hwloc_constructor(void)
 {