src/test/perl/TestLib.pm: check_pg_config needs/usr/include/postgresql/pg_config.h

I believe check_pg_config as used by src/test/ssl/t/002_scram.pl
shouldn't rely on /usr/include/postgresql/pg_config.h but use the file
from the build tree instead:

src/test/perl/TestLib.pm:

    Return the number of matches of the given regular expression
    within the installation's C<pg_config.h>.

    =cut

    sub check_pg_config
    {
        my ($regexp) = @_;
        my ($stdout, $stderr);
        my $result = IPC::Run::run [ 'pg_config', '--includedir' ], '>',
          \$stdout, '2>', \$stderr
          or die "could not execute pg_config";
        chomp($stdout);
        $stdout =~ s/\r$//;

        open my $pg_config_h, '<', "$stdout/pg_config.h" or die "$!"; <-- here
        my $match = (grep { /^$regexp/ } <$pg_config_h>);
        close $pg_config_h;
        return $match;
    }

src/test/ssl/README claims that it is possible to run the "ssl" extra
test from make check (as opposed to installcheck).

Christoph