See: https://savannah.nongnu.org/bugs/?58606
and: mpfr-4.1.1/tests/tcmp_ui.c
The code '__builtin_constant_p ((i++, 7))' was not working.
I Fixed it in tccgen.c
I added a testcase in tests/tcctest.c. I also wanted to add a test like
'__builtin_constant_p ((10, 7))' but gcc needs -O1 for that to work.
clang (and now tcc) work as expected.