Only check for windres during cross-builds

windres is not needed for non-Windows builds and should
therefore only be checked if we are cross-compiling.
Otherwise warnings or errors could confuse the user.

ac/initialization.m4

@@ -13,7 +13,6 @@ AC_CHECK_TOOL(RANLIB, ranlib, :)
 AC_CHECK_TOOL(STRIP, strip, :)
 AC_CHECK_TOOL(AR, ar, :)
 AC_CHECK_TOOL(LD, ld, :)
-AC_CHECK_TOOL(WINDRES, windres, :)
 
 dnl Check for headers
 AC_HEADER_STDC()

ac/mingw.m4

@@ -14,6 +14,8 @@ AC_CACHE_CHECK([if being compiled with mingw32],
     MINGW_GUIAPP=-mwindows
     LIBMTXCOMMONDLL=1
     EXEEXT=.exe
+
+    AC_CHECK_TOOL(WINDRES, windres, :)
   else
     LIBMTXCOMMONDLL=0
   fi