Fix all instances of '#if HAVE_CONFIG_H'.

Should be '#ifdef HAVE_CONFIG_H'.

Closes: https://sourceforge.net/p/flac/bugs/410/
diff --git a/src/libFLAC/float.c b/src/libFLAC/float.c
index f92a9b1..463cf9d 100644
--- a/src/libFLAC/float.c
+++ b/src/libFLAC/float.c
@@ -30,7 +30,7 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#if HAVE_CONFIG_H
+#ifdef HAVE_CONFIG_H
 #  include <config.h>
 #endif