configure.ac: unbreak the build with non gnu grep

configure.ac: unbreak the build with non gnu grep

181581280bd430d122d416e308c1de82db82da04 changed the way the
llvm-config version is read from sed to grep and introduced
a requirement for gnu grep extension that treats BREs as EREs.

Avoid this by calling egrep instead of grep which should be
able to handle EREs everywhere.

This allows Mesa to build on OpenBSD again.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>