The add_external_project function separates its arguments with ';'
separators, so previously no command line argument could contain one.
When specifying CMAKE_ARGS, some -D argument values may need to contain
a semicolon to form lists in the external project cache.

This adds add_external_project argument LIST_SEPARATOR to specify a list
separator string.  The separator is replaced by ';' in arguments to any
command created to drive the external project.  For example:

  add_external_project(...
    LIST_SEPARATOR ::
    CMAKE_ARGS -DSOME_LIST:STRING=A::B::C
    ...)

passes "-DSOME_LIST:STRING=A;B;C" to CMake for the external project.