Avoid PCRE syntax when using grep
Avoid PCRE syntax when using grep because some versions of grep don't support that.
This avoids the erroneous error message...
====================================================================================== Meson version does not seem to match autotools version 2.13.94, update meson.build! ======================================================================================
...that I saw when building fontconfig 2.13.94 (with other patches) on Mac OS X 10.4.