unittest-cpp: fix 'Version:' field in .pc file

Before the change project built using `cmake` had Version field empty:

    Version:

After the change version is set as expected:

    Version: 2.1.0

Noticed as as a build failure of `echant-2.6.4` which failed to find the
package using `pkg-config --exists --print-errors "UnitTest++ >= 1.6"`
expression.

pkgs/development/libraries/unittest-cpp/default.nix

@@ -24,6 +24,10 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  # Fix 'Version:' setting in .pc file. TODO: remove once upstreamed:
+  #     https://github.com/unittest-cpp/unittest-cpp/pull/188
+  cmakeFlags = [ "-DPACKAGE_VERSION=${version}" ];
+
   nativeBuildInputs = [ cmake ];
 
   doCheck = false;