Call SetDllDirectory("") after sqlite3 init on cygwin

Cygwin sqlite3 is patched to call SetDllDirectory("/usr/bin") on init, which
affects the current process and is inherited by child processes.  It causes
DLLs to be loaded from /usr/bin/ before $PATH, which breaks all sorts of
things.  A typical failures would be header/lib version mismatches (e.g.
openssl when running checkPhase on openssh).  We'll just set it back to the
default value.

Note that this is a problem with the cygwin version of sqlite3 (currently
3.18.0).  nixpkgs doesn't have the problematic patch.

src/libstore/local-store.cc

@@ -30,6 +30,10 @@
 #include <sys/xattr.h>
 #endif
 
+#ifdef __CYGWIN__
+#include <windows.h>
+#endif
+
 #include <sqlite3.h>
 
 
@@ -281,6 +285,16 @@ void LocalStore::openDB(State & state, bool create)
             SQLITE_OPEN_READWRITE | (create ? SQLITE_OPEN_CREATE : 0), 0) != SQLITE_OK)
         throw Error(format("cannot open Nix database ‘%1%’") % dbPath);
 
+#ifdef __CYGWIN__
+    /* The cygwin version of sqlite3 has a patch which calls
+       SetDllDirectory("/usr/bin") on init. It was intended to fix extension
+       loading, which we don't use, and the effect of SetDllDirectory is
+       inherited by child processes, and causes libraries to be loaded from
+       /usr/bin instead of $PATH. This breaks quite a few things (e.g.
+       checkPhase on openssh), so we set it back to default behaviour. */
+    SetDllDirectoryW(L"");
+#endif
+
     if (sqlite3_busy_timeout(db, 60 * 60 * 1000) != SQLITE_OK)
         throwSQLiteError(db, "setting timeout");