install back the safety check of includes.test

Compilation of each header file alone, as a safety check, was removed
when introducing "#pragma once" because we did not have to check for
possible double inclusion.  However we still need to compile each
header to make sure they are self-contained.

* tests/sanity/includes.test: Compile each header.
* tests/run.in: Export various compiler and directory flags.
* spot/twaalgos/emptiness_stats.hh, spot/misc/mspool.hh,
spot/misc/fixpool.hh: Include <spot/misc/common.hh>.
* spot/misc/common.hh: Include <cassert>.
* NEWS: Mention the fixed headers.

spot/misc/fixpool.hh

@@ -19,10 +19,10 @@
 
 #pragma once
 
+#include <spot/misc/common.hh>
 #include <new>
 #include <cstddef>
 #include <cstdlib>
-#include <cassert>
 
 namespace spot
 {