ExternalProject: Fix regression in passing list to CMAKE_CACHE_ARGS

Fix passing a list to the CMAKE_CACHE_ARGS and CMAKE_CACHE_DEFAULT_ARGS
options of ExternalProject_Add.

Following commit v3.7.0-rc1~273^2~1 (prefer list(APPEND) over
string(APPEND) where appropriate, 2016-08-08), the semicolon list
separator after the first list element was missing in the generated
cache.

Modules/ExternalProject.cmake

@@ -1160,7 +1160,7 @@ function(_ep_command_line_to_initial_cache var args force)
       endif()
     else()
       # Assume this is a list to append to the last var
-      list(APPEND accumulator "${line}")
+      string(APPEND accumulator ";${line}")
     endif()
   endforeach()
   # Catch the final line of the args

Tests/RunCMake/ExternalProject/CMAKE_CACHE_ARGS-check.cmake

@@ -15,3 +15,12 @@ if(NOT "${CMAKE_MATCH_0}" MATCHES FORCE)
   set(RunCMake_TEST_FAILED "Expected forced FOO argument")
   return()
 endif()
+
+if(NOT "${_cache}" MATCHES "set\\(TEST_LIST \"A;B;C\".+\\)")
+  set(RunCMake_TEST_FAILED "Cannot find TEST_LIST argument in cache")
+  return()
+endif()
+if(NOT "${CMAKE_MATCH_0}" MATCHES FORCE)
+  set(RunCMake_TEST_FAILED "Expected forced TEST_LIST argument")
+  return()
+endif()

Tests/RunCMake/ExternalProject/CMAKE_CACHE_ARGS.cmake

@@ -5,4 +5,6 @@ include(ExternalProject)
 
 ExternalProject_Add(FOO TMP_DIR "${CMAKE_CURRENT_BINARY_DIR}/tmp"
                         DOWNLOAD_COMMAND ""
-                        CMAKE_CACHE_ARGS "-DFOO:STRING=$<1:BAR>$<0:BAD>")
+                        CMAKE_CACHE_ARGS
+                            "-DFOO:STRING=$<1:BAR>$<0:BAD>"
+                            "-DTEST_LIST:STRING=A;B;C")

Tests/RunCMake/ExternalProject/CMAKE_CACHE_DEFAULT_ARGS-check.cmake

@@ -15,3 +15,12 @@ if("${CMAKE_MATCH_0}" MATCHES FORCE)
   set(RunCMake_TEST_FAILED "Expected not forced FOO argument")
   return()
 endif()
+
+if(NOT "${_cache}" MATCHES "set\\(TEST_LIST \"A;B;C\".+\\)")
+  set(RunCMake_TEST_FAILED "Cannot find TEST_LIST argument in cache")
+  return()
+endif()
+if("${CMAKE_MATCH_0}" MATCHES FORCE)
+  set(RunCMake_TEST_FAILED "Expected not forced TEST_LIST argument")
+  return()
+endif()

Tests/RunCMake/ExternalProject/CMAKE_CACHE_DEFAULT_ARGS.cmake

@@ -5,4 +5,6 @@ include(ExternalProject)
 
 ExternalProject_Add(FOO TMP_DIR "${CMAKE_CURRENT_BINARY_DIR}/tmp"
                         DOWNLOAD_COMMAND ""
-                        CMAKE_CACHE_DEFAULT_ARGS "-DFOO:STRING=$<1:BAR>$<0:BAD>")
+                        CMAKE_CACHE_DEFAULT_ARGS
+                            "-DFOO:STRING=$<1:BAR>$<0:BAD>"
+                            "-DTEST_LIST:STRING=A;B;C")