Qt: Make sure extended grep is used with '|'.

* m4/qt.m4: Use grep -E when using the alternation character.

POSIX specifies '|' is only supposed to work as an alternation special
character when grep is used in extended mode. The code worked fine
with GNU grep because it accepts extended regular expressions by
default, but other POSIX-compliant implementations might fail and take
it literally.

• Signed-off-by: Raphael Kubo da Costa <rakuco@FreeBSD.org>