CBMC does not build with clang on ubuntu 22.04

[esteffin]

Building CBMC with clang on ubuntu 22.04 fails.

The issue is that during the compilation of target library-check.stamp at step Checking /root/cbmc/src/ansi-c/library/stdio.c there is an error:

__libcheck.c:859:5: error: definition with same mangled name '__isoc99_fscanf' as another definition
int __isoc99_fscanf

The issue happens with clang-11, clang-12 and clang-13.

The build using gcc instead works.

CBMC version: commit d932d6f
Operating system: Ubuntu 22.04
Exact command line resulting in the issue: make
What behaviour did you expect: A correct build
What happened instead: Error during build

[esteffin]

After a bit of investigation it seems that the script src/ansi-c/library_check.sh fails when checking stdio.c as clang cannot build it on Ubuntu 22.04.

On Mac the compilation with clang-12 instead is successful.

[tautschnig]

We should probably #ifndef the non __isoc99_ variants when __clang__ is set.

[tautschnig]

To be fixed as part of #7496.