"Missing C library: pq" - can't build with stack on windows

[joozek78]

• I have both pg_config and libpq.dll in my build directory
• My project builds fine under cabal
• with stack it fails with

Configuring postgresql-libpq-0.9.0.2...
    Setup.hs: Missing dependency on a foreign library:
    * Missing C library: pq
    This problem can usually be solved by installing the system package that
    provides this library (you may need the "-dev" version). If the library is
    already installed but in a non-standard location then you can use the flags
    --extra-include-dirs= and --extra-lib-dirs= to specify where it is.