Make libpq thread-safe with configure --with-threads option.

configure

configure.in

src/interfaces/libpq/fe-auth.c

src/interfaces/libpq/fe-connect.c

src/interfaces/libpq/fe-lobj.c

src/interfaces/libpq/fe-misc.c

src/interfaces/libpq/fe-secure.c

src/interfaces/libpq/libpq-int.h

src/interfaces/libpq/win32.c

src/interfaces/libpq/win32.h