libpq parallel build

src/interfaces/libpq/fe-misc.c #include-s pg_config_paths.h, but in
the makefile that dependency is not declared.

This breaks the build with 'make -jN' (with a "big" N) ... sometimes
(non-deterministically).

Patch attached. Made from 9.1.1, but from browsing the git master on
the web interface, the facts above seem to still be true.

-- 
Lionel