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.

tests/run.in

@@ -52,6 +52,15 @@ srcdir="@srcdir@"
 # want to inherit parameters likes -std=c11 -fvisibility=hidden
 CFLAGS=
 export CFLAGS
+CXX='@CXX@'
+export CXX
+CXXFLAGS='@CXXFLAGS@'
+export CXXFLAGS
+top_builddir='@abs_top_builddir@'
+export top_builddir
+top_srcdir='@abs_top_srcdir@'
+export top_srcdir
+
 
 case $1 in
     */*)