Revert "Properly detect POSIX threads on OpenBSD."

If main() is not found in pthread 10 line above it won't be find now.
And the patch submitter is looking for a better test

This reverts commit bb1d1eb5.

configure.ac

@@ -825,9 +825,6 @@ fi
 if test "${THREAD_LIB}" = "error"; then
   AC_CHECK_LIB(c_r,main,THREAD_LIB="-lc_r")
 fi
-if test "${THREAD_LIB}" = "error"; then
-  AC_CHECK_LIB(pthread,main,THREAD_LIB="-pthread")
-fi
 if test "${THREAD_LIB}" = "error"; then
   AC_CHECK_FUNCS(pthread_mutex_lock)
   THREAD_LIB=""