FindFLEX: Add policy CMP0098 to run flex in build tree

Original Description

The two top commits help

• create files generated by flex (scanner file, header file, external serialized file) fin the current binary directory, by default ('VPATH build')
  • to not pollute the source directory with a generated file
  • to allow build-specific generated files without confusing with generated files from other builds
• ensure that the run-time dependent serialized file is build-specific and resides next to the .c and hence the .exe file, by default
• monitor dependencies on a build-specific serialized tables file (using the new option TABLES_FILE)

Hope the commits satisfy your commit policy requests.

Topic-rename: FindFLEX-work-dir