Further restrict the scope of no-exit()-in-libpq test.
Disable this check altogether in --enable-coverage builds,
because newer versions of gcc insert exit() as well as abort()
calls for that. Also disable it on AIX and Solaris, because
those platforms tend to provide facilities such as libldap
as static libraries, which then get included in libpq's shlib.
We can't expect such libraries to honor our coding rules.
(That platform list might need additional tweaking, but I think
this is enough to keep the buildfarm happy.)
Per reports from Jacob Champion and Noah Misch.
Discussion: https://postgr.es/m/
3128896.
1624742969@sss.pgh.pa.us