GetPrerequisites: Optionally filter "objdump" output for speed

As dumpbin.exe is no longer reliable for gcc libraries on MinGW because
it crashes on many common libraries like libgcc_s and libgfortran it is
now necessary too resort to using objdump for DLL dependency walking.
Using objdump has a secondary problem in that it generates a lot of
output for large libraries and causes fixup_bundle() to take many
minutes to process what took fractions of a second with
"dumpbin.exe /dependents".

Add a 'grep' pre-filter in the execute_process() command pipeline to
reduce this output to a minimum for a several orders of magnitude speed
up.  If grep is not available just use the full output.

As there does not seem to be a reliable way of detecting MinGW, callers
of fixup_bundle() may have to set the variable gp_tool to "objdump" if
dumpbin.exe is installed on the build machine to stop it using the
broken MS dumpbin.exe for library dependency walking.

Modules/GetPrerequisites.cmake

@@ -700,6 +700,8 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
     return()
   endif()
 
+  set(gp_cmd_maybe_filter)      # optional command to pre-filter gp_tool results
+
   if(gp_tool STREQUAL "ldd")
     set(gp_cmd_args "")
     set(gp_regex "^[\t ]*[^\t ]+ => ([^\t\(]+) .*${eol_char}$")
@@ -724,6 +726,11 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
     set(gp_regex_error "")
     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(gp_grep_cmd)
+      set(gp_cmd_maybe_filter COMMAND ${gp_grep_cmd} "^[[:blank:]]*DLL Name: ")
+    endif()
   else()
     message(STATUS "warning: gp_tool='${gp_tool}' is an unknown tool...")
     message(STATUS "CMake function get_prerequisites needs more code to handle '${gp_tool}'")
@@ -780,6 +787,7 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
   #
   execute_process(
     COMMAND ${gp_cmd} ${gp_cmd_args} ${target}
+    ${gp_cmd_maybe_filter}
     RESULT_VARIABLE gp_rv
     OUTPUT_VARIABLE gp_cmd_ov
     ERROR_VARIABLE gp_ev