Genex: Teach SHELL_PATH to support a list of paths
Extend the genex added by commit ca6ba3fe to accept a ;
-list of paths, convert them all, and generate a list separated by the native shell `PATH``` separator.
In $<SHELL_PATH:...>
the parameter can now also be a list of paths in which case the paths will be separated by the OS dependent path separator. On linux the path separator is ':'
and on Windows it is ';'
. If the parameter is just one path the behaviour is identical to the current $<SHELL_PATH:...>