ulogd: use AC_SEARCH_LIBS for libpthread

Some uClibc-based toolchains lack threading support, so use
AC_SEARCH_LIB instead of AC_CHECK_LIB to check for libpthread
availability and link conditionally if found since it's only used for
the database backends.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>