configure.ac: move pthread check earlier
configure.ac: move pthread check earlier

The time checks will need to know if pthread exists in
a future commit.

git-svn-id: ae38cf1093b87151f738d2b2220cac7742ad2e55@844 64e312b2-a51f-0410-8e61-82d0ca0eb02a

lubosz/check,lubosz/check,lubosz/check,lubosz/check