libssh2: fix shared library build under mingw

pkgs/development/libraries/libssh2/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurlBoot, openssl, zlib}:
+{stdenv, fetchurlBoot, openssl, zlib, windows}:
 
 stdenv.mkDerivation rec {
   name = "libssh2-1.6.0";
@@ -10,6 +10,21 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ openssl zlib ];
 
+  crossAttrs = {
+    # link against cross-built libraries
+    configureFlags = [
+      "--with-openssl"
+      "--with-libssl-prefix=${openssl.crossDrv}"
+      "--with-libz"
+      "--with-libz-prefix=${zlib.crossDrv}"
+    ];
+  } // stdenv.lib.optionalAttrs (stdenv.cross.libc == "msvcrt") {
+    # mingw needs import library of ws2_32 to build the shared library
+    preConfigure = ''
+      export LDFLAGS="-L${windows.mingw_w64}/lib $LDFLAGS"
+    '';
+  };
+
   meta = {
     description = "A client-side C library implementing the SSH2 protocol";
     homepage = http://www.libssh2.org;