Enable WIN32 compilation of libpq.

src/backend/utils/mb/common.c

src/interfaces/libpq/fe-auth.c

src/interfaces/libpq/fe-connect.c

src/interfaces/libpq/fe-exec.c

src/interfaces/libpq/fe-lobj.c

src/interfaces/libpq/fe-misc.c

src/interfaces/libpq/fe-print.c