win32: only check for windres for win32, and fail if missing

configure.ac

@@ -125,7 +125,6 @@ AC_PROG_MKDIR_P
 AC_PROG_SED
 AC_PATH_TOOL(AR, ar)
 AC_PATH_TOOL(RANLIB, ranlib)
-AC_PATH_TOOL(WINDRES, windres)
 AC_PATH_TOOL(STRIP, strip)
 AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_PROG(LCOV, lcov)
@@ -183,6 +182,11 @@ case $host in
        AC_MSG_WARN("makensis not found. Cannot create installer.")
      fi
 
+     AC_PATH_TOOL(WINDRES, windres, none)
+     if test x$WINDRES = xnone; then
+       AC_MSG_ERROR("windres not found")
+     fi
+
      CPPFLAGS="$CPPFLAGS -D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB"
      LEVELDB_TARGET_FLAGS="TARGET_OS=OS_WINDOWS_CROSSCOMPILE"
      CXXFLAGS="$CXXFLAGS -w"