find_program: Optionally consider all names in each directory

When more than one value is given to the NAMES option this command by
default will consider one name at a time and search every directory for
it.  Add a NAMES_PER_DIR option to tell this command to consider one
directory at a time and search for all names in it.

Source/cmFindProgramCommand.h

@@ -25,6 +25,7 @@
 class cmFindProgramCommand : public cmFindBase
 {
 public:
+  cmFindProgramCommand();
   /**
    * This is a virtual constructor for the command.
    */
@@ -55,6 +56,8 @@ public:
 private:
   std::string FindProgram();
   std::string FindNormalProgram();
+  std::string FindNormalProgramDirsPerName();
+  std::string FindNormalProgramNamesPerDir();
   std::string FindAppBundle();
   std::string GetBundleExecutable(std::string bundlePath);