Add a --enable-pthread option to activate experimental threading code

* NEWS, README, doc/org/compile.org: Mention the option and
its effect on compilation requirements.
* configure.ac: Add the --enable-pthread option, and ENABLE_PTHREAD
macro.
* doc/org/g++wrap.in, spot/Makefile.am, spot/libspot.pc.in: Compile
with -pthread conditionally.
* spot/graph/graph.hh, spot/twa/twagraph.cc: Adjust the code to not
use thread-local variables, and let the pthread code be optional.
* .gitlab-ci.yml: Activate --enable-pthread in two configurations.

doc/org/compile.org

@@ -210,11 +210,14 @@ one library requiring another, you will need to link with the =bddx=
 library.  This should be as simple as adding =-lbddx= after =-lspot=
 in the first three cases.
 
+Similarly, if Spot has been configured with =--enable-pthread=, you
+will need to add =-pthread= to the compiler flags.
+
 In the fourth case where =libtool= is used to link against
 =libspot.la= linking against =libbddx.la= should not be necessary because
 Libtool already handles such dependencies.  However the version of =libtool=
 distributed with Debian is patched to ignore those dependencies, so in this
-case you 2
+case you have to list all dependencies.
 
 * Additional suggestions
 

doc/org/g++wrap.in

@@ -1,6 +1,6 @@
 #!/bin/sh
 # This is a wrapper around the compiler, to ensure that the code
-# example run from the org-mode file are all linked with Spot.
+# examples run from org-mode files are all linked with Spot.
 #
 # Also we save errors to org.errors, so that we can detect issues
 # after org-mode has exported everything.  Otherwise these errors
@@ -8,7 +8,7 @@
 @top_builddir@/libtool link @CXX@ @CXXFLAGS@ @CPPFLAGS@ -Wall -Werror \
  -I@abs_top_builddir@ -I@abs_top_srcdir@ -I@abs_top_srcdir@/buddy/src \
  "$@" @abs_top_builddir@/spot/libspot.la \
-      @abs_top_builddir@/buddy/src/libbddx.la 2> errors.$$
+      @abs_top_builddir@/buddy/src/libbddx.la @LIBSPOT_PTHREAD@ 2> errors.$$
 code=$?
 if test $code -ne 0 && test -s errors.$$; then
     cat errors.$$ >>org.errors