Minor 8.1.0 compilation issues on linux

[yuriy-yarosh]

Describe the bug

Building 8.1.0 from source code on Archlinux with LLVM and stock (non su2) meson/ninja.
I've installed python 3.13 with asdf and bundled meson.py script does not resolve the ninja PATH correctly.

While trying to build the code I've stumbled upon couple issues

1. Posix definition for strict fileno usage
   externals/cgns/adf/ADF_internals.c requires #define _POSIX_SOURCE before including stdio.h for fileno implicit function definition to actually be resolved, over here tbe

2. Missing strings.h include in Metis externals/metis/GKlib/gkregex.c for strcasecmp to actually be resolved, over here tbe

Desktop (please complete the following information):

• OS: Arch
• C++ compiler and version: llvm18.1.8 Python 3.13.1 ninja 1.12.1 meson 1.6.1 with asdf
• MPI implementation and version: arch OpenMPI 5.0.6
• SU2 Version: v8.1.0

[pcarruscag]

Thanks, let us know when the CGNS and METIS developers give you an update.