CMake: handle multiple paths stored in XXX__INCLUDE_DIR when checking for version strings

[jmckenna]

Feature description

Packagers must often specify multiple paths inside a single CMake variable, such as for multiple includes, or libs, for a single driver.

However GDAL's CMake system assumes that the variable contains only one value (path), when looking for version strings. For example:

• in FindLibKML.cmake see the line
  • file(STRINGS ${LIBKML_INCLUDE_DIR}/kml/base/version.h libkml_version_h_string
• a packager might have set LIBKML_INCLUDE_DIR = "LIB1/include;LIB1/install/include;LIB3/include"
• that causes warnings of
  • LIBXXX has unknown version. Assuming it is at least matching the minimum version required of X.X

Additional context

I've been modifying the FindXXX.cmake paths for the version strings by hand for many GDAL drivers, but I am sure that it would be beneficial to all packagers if this was handled somehow out of the box. thanks.