Fix "gmake check" under OpenBSD 6.3 (probably *BSD): Avoid using GNU grep specific regexp handling #13355

Fixes #13337 (!)

GNU grep and BSD grep differs in the way they handle regexps when extended regular expressions are not enabled via the -E flag:

$ grep --version | head -1
grep (GNU grep) 3.1
$ echo "BOOST_FIXTURE_TEST_SUITE(foo)" | grep "BOOST_FIXTURE_TEST_SUITE(\|BOOST_AUTO_TEST_SUITE("
BOOST_FIXTURE_TEST_SUITE(foo)
$

$ grep --version | head -1
grep version 0.9
$ echo "BOOST_FIXTURE_TEST_SUITE(foo)" | grep "BOOST_FIXTURE_TEST_SUITE(\|BOOST_AUTO_TEST_SUITE("
$ 

The portable way to do it is:

$ echo "BOOST_FIXTURE_TEST_SUITE(foo)" | grep -E "(BOOST_FIXTURE_TEST_SUITE\\(|BOOST_AUTO_TEST_SUITE\\()"
BOOST_FIXTURE_TEST_SUITE(foo)
$

utACK db56755ca4a0e53785b5bf322d3b65ffe328b60a

(can we get a Tested ACK from you @grim-trigger?)

Tested ACK db56755

Works fine on OpenBSD 6.3/AMD64

tACK db56755 on OpenBSD 6.3

(cf #12926)