Best/Shortest way to join a list in CMake

You can use the string REPLACE function:

function(JOIN VALUES GLUE OUTPUT)
  string (REPLACE ";" "${GLUE}" _TMP_STR "${VALUES}")
  set (${OUTPUT} "${_TMP_STR}" PARENT_SCOPE)
endfunction()

#USAGE:
SET(somelist a b c)
JOIN("${somelist}" ":" output)
MESSAGE("${output}") # will output "a:b:c"

Usually this task is solved with simple string REPLACE command. You can find a number of such replaces in scripts coming with cmake. But if you really need to care about semicolons inside you values, then you can use the following code:

function(JOIN VALUES GLUE OUTPUT)
  string (REGEX REPLACE "([^\\]|^);" "\\1${GLUE}" _TMP_STR "${VALUES}")
  string (REGEX REPLACE "[\\](.)" "\\1" _TMP_STR "${_TMP_STR}") #fixes escaping
  set (${OUTPUT} "${_TMP_STR}" PARENT_SCOPE)
endfunction()

SET( letters "" "\;a" b c "d\;d" )
JOIN("${letters}" ":" output)
MESSAGE("${output}") # :;a:b:c:d;d

CMake 3.12 is poised to finally add this upstream:

https://github.com/Kitware/CMake/commit/a58158727be4585f9fd71449e9cc9e801c59a009

https://github.com/Kitware/CMake/commit/689eeb67cb87a9ed1d91a4971806488d00e68f42