Teach find_(path|file) about Linux multiarch (#13742)

Implement support for multiarch include directories as specified here:

  https://wiki.ubuntu.com/MultiarchCross

Generalize the multiarch feature added in commit b41ad3b3 (Teach
find_(library|package) about Linux multiarch, 2011-06-08) to the
find_path and find_file commands.  Teach them to search
<prefix>/include/<arch> whenever they would search <prefix>/include.

Source/cmFindBase.cxx

@@ -360,13 +360,13 @@ void cmFindBase::AddPrefixPaths(std::vector<std::string> const& in_paths,
       {
       dir += "/";
       }
-    if(subdir == "lib")
+    if(subdir == "include" || subdir == "lib")
       {
       const char* arch =
         this->Makefile->GetDefinition("CMAKE_LIBRARY_ARCHITECTURE");
       if(arch && *arch)
         {
-        this->AddPathInternal(dir+"lib/"+arch, pathType);
+        this->AddPathInternal(dir+subdir+"/"+arch, pathType);
         }
       }
     std::string add = dir + subdir;

Source/cmFindPathCommand.cxx

@@ -45,8 +45,10 @@ void cmFindPathCommand::GenerateDocumentation()
                                "SEARCH_XXX", "file in a directory");
   cmSystemTools::ReplaceString(this->GenericDocumentation,
                                "XXX_SUBDIR", "include");
-  cmSystemTools::ReplaceString(this->GenericDocumentation,
-                               "XXX_EXTRA_PREFIX_ENTRY", "");
+  cmSystemTools::ReplaceString(
+    this->GenericDocumentation,
+    "XXX_EXTRA_PREFIX_ENTRY",
+    "   <prefix>/include/<arch> if CMAKE_LIBRARY_ARCHITECTURE is set, and\n");
   cmSystemTools::ReplaceString(this->GenericDocumentation,
                                "CMAKE_FIND_ROOT_PATH_MODE_XXX",
                                "CMAKE_FIND_ROOT_PATH_MODE_INCLUDE");