Fixes RDK_BUILD_THREADSAFE_SSS=OFF build (#4349)

* - fix non-threaded *nix builds that currently fail because
  boost flyweight introduces a dependency on pthreads
- make sure that mutexes and futures are only used when
  RDK_BUILD_THREADSAFE_SSS is ON
- fix SubstructMatch failing test when RDK_BUILD_THREADSAFE_SSS is OFF
  due to misplaced #ifdef's
- rename RDK_TEST_MULTITHREADED to RDK_THREADSAFE_SSS in inchi.cpp
  (which is not a test)

* - the limitexternal Linux build is now single-threaded so we make
  sure single-threaded builds do not break in the future
  (suggestion from Greg)

* reverted unnecessary change to Code/GraphMol/FileParsers/testMultithreadedMolSupplier.cpp

Co-authored-by: Paolo Tosco <paolo.tosco@novartis.com>

External/YAeHMOP/EHTTools.cpp

@@ -4,7 +4,9 @@
 #include "EHTTools.h"
 #include <GraphMol/RDKitBase.h>
 #include <GraphMol/MolOps.h>
+#ifdef RDK_THREADSAFE_SSS
 #include <mutex>
+#endif
 #include <fstream>
 
 extern "C" {
@@ -19,11 +21,15 @@ const std::string _EHTChargeMatrix = "_EHTChargeMatrix";
 
 // we should only call into the C code, which uses tons of globals, from one
 // thread at a time. This mutex enforces that.
+#ifdef RDK_THREADSAFE_SSS
 std::mutex yaehmop_mutex;
+#endif
 
 bool runMol(const ROMol &mol, EHTResults &results, int confId,
             bool preserveHamiltonianAndOverlapMatrices) {
+#ifdef RDK_THREADSAFE_SSS
   std::lock_guard<std::mutex> lock(yaehmop_mutex);
+#endif
 
   // -----------------------------
   // ----- BOILERPLATE -----------