GetPrerequisites: Optimize on Windows by filtering `objdump` output

Extend the optimization from commit v3.4.0-rc1~264^2 (GetPrerequisites:
Optionally filter "objdump" output for speed, 2015-07-29) to work
on Windows by using `findstr` in place of `grep`.

Modules/GetPrerequisites.cmake

@@ -747,7 +747,11 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
     set(gp_regex_fallback "")
     set(gp_regex_cmp_count 1)
     # objdump generaates copious output so we create a grep filter to pre-filter results
-    find_program(gp_grep_cmd grep)
+    if(WIN32)
+      find_program(gp_grep_cmd findstr)
+    else()
+      find_program(gp_grep_cmd grep)
+    endif()
     if(gp_grep_cmd)
       set(gp_cmd_maybe_filter COMMAND ${gp_grep_cmd} "^[[:blank:]]*DLL Name: ")
     endif()