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.