pgsql-server: Be more aggressive about adding flags to thread compiles.

Log Message:
-----------
Be more aggressive about adding flags to thread compiles.  The configure
test only tests for building a binary, not building a shared library.

On Linux, you can build a binary with -pthread, but you can't build a
binary that uses a threaded shared library unless you also use -pthread
when building the binary, or adding -lpthread to the shared library
build.  This patch has the effect of doing the later by adding both
-pthread and -lpthread when building libpq.

Modified Files:
--------------
    pgsql-server/config:
        acx_pthread.m4 (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/config/acx_pthread.m4.diff?r1=1.3&r2=1.4)