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.

README

@@ -173,6 +173,12 @@ flags specific to Spot:
      client code should be compiled with -D_GLIBCXX_DEBUG as well.  This
      options should normally only be useful to run Spot's test-suite.
 
+   --enable-pthread
+     Build and link with the -pthread option, and activate a few
+     parallel variants of the algorithms.  This is currently disabled
+     by default, as it require all third-party tools using Spot to
+     build with -pthread as well.
+
    --enable-c++20
      Build everything in C++20 mode.  We use that in our build farm to
      ensure that Spot can be used in C++20 projects as well.