Make cmake.h available everywhere.

The definition of HAVE_CMAKE_H is now at the very top of the main
CMakeLists.txt, so that it applies to all objects. And the consequent
include of cmake.h is at the very top of defs.h, so that it should be
included first by everything. This way, I don't have to worry any more
that the HAVE_FOO definitions in cmake.h might accidentally have
failed to reach some part of the code.

defs.h

@@ -22,6 +22,10 @@
 #error Do not compile this code base with NDEBUG defined!
 #endif
 
+#if HAVE_CMAKE_H
+#include "cmake.h"
+#endif
+
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>                     /* for __MINGW_PRINTF_FORMAT */