COMP: suppress false positives from -Wdangling-reference (gcc)

- the number of false positives has reduced, but in a few remaining
  cases, the compiler cannot possibly "know" that the pointer stored
  on the registry will outlive the scope of the method (for example)

  gcc-13: suppressing these spurious warnings needs to be done at each
  caller, which is simply not worth it, since later compiler versions
  provide a cleaner solution.

  gcc-14: the [[gnu::no_dangling]] attribute on the declaration
  lets the compiler know the intent.

- additional FOAM_REAL_GNUC macro (defined in stdFoam.H) to define
  the "real" gcc version and ignoring other compilers masquerading
  as gcc.

COMP: rename MeshObject code to {cxx,txx}

- avoids issues on case-insensitive filesystems (#3316)

applications/utilities/parallelProcessing/decomposePar/decomposePar.C

@@ -210,6 +210,7 @@ autoPtr<labelIOList> faProcAddressing
 
 
 // Return cached or read proc addressing from facesInstance
+FOAM_NO_DANGLING_REFERENCE
 const labelIOList& procAddressing
 (
     const PtrList<fvMesh>& procMeshList,