original definitions took advantage of the non-compliant behavior
wrt decimal integer litteral

These tests have missing specs, so the large amount of warnings is expected.

There are still many missing specifications, but at least parsing should
succeed for strictly POSIX-compliant code.

Some scripts use features which cannot or should not be tested
(e.g. reliance on an optional external tool); defining PTESTS_TESTING=1
allows these scripts to detect such circumstances and change (slightly)
their behavior to avoid non-reproducible oracles.

Frama-C does not yet support _Alignof and _Alignas, but this header helps
parsing code and emitting a proper error message.

The ambiguous regex led to behavioral differences under a musl-based libc,
such as in Frama-C's Docker images. The new regex is longer, but avoids the
issue.