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.
e48322a6d6