haskellPackages.postgresql-libpq: Use pkg-config instead of pg_config…

… to find libpq

During configure, postgresql-libpq's Setup.hs will try to execute pg_config to
find libpq. This will not work when cross-compiling, because pg_config was compiled
for the target system, but needs to run on the host.

The "use-pkg-config" flag allows to do this via pkg-config instead, which works
better in those cases.

pkgs/development/haskell-modules/configuration-nix.nix

@@ -1379,4 +1379,12 @@ self: super: builtins.intersectAttrs super {
     mpi-hs-cereal
     mpi-hs-binary
     ;
+
+  postgresql-libpq = overrideCabal (drv: {
+    # Using use-pkg-config flag, because pg_config won't work when cross-compiling.
+    configureFlags = drv.configureFlags or [] ++ [ "-fuse-pkg-config" ];
+    # Move postgresql from SystemDepends to PkgconfigDepends
+    libraryPkgconfigDepends = drv.librarySystemDepends;
+    librarySystemDepends = [];
+  }) super.postgresql-libpq;
 }