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