replace #if HAVE_CONFIG_H by #ifdef HAVE_CONFIG_H

it seems like most build systems pass a -DHAVE_CONFIG_H flag to the
compiler which still causes `#if HAVE_CONFIG_H` to be false while it
clearly is supposed to be triggered.

That said, I do not really see a good reason why the inclusion of the
`config.h` file should be guarded in the first place: the file is
guaranteed to always available by proper build systems, and if it was
not included the build either breaks at the linking stage or -- at the
very least -- the runtime behavior of the resulting libraries will be
very awkward.

examples/compute_initial_state.cpp

@@ -19,7 +19,7 @@
 */
 
 
-#if HAVE_CONFIG_H
+#ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif // HAVE_CONFIG_H