ENH: Allow lists in AddExternalProject arguments

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.

Tests/ExternalProject/CMakeLists.txt

@@ -117,16 +117,20 @@ endif()
 set(proj TutorialStep1-LocalTAR)
 add_external_project(${proj}
   TAR "${CMAKE_CURRENT_SOURCE_DIR}/Step1.tar"
+  LIST_SEPARATOR ::
   PATCH_COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/Step1Patch.cmake
   CMAKE_GENERATOR "${CMAKE_GENERATOR}"
   CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=${prefix}
+             -DTEST_LIST:STRING=A::B::C
   INSTALL_COMMAND ""
 )
 
 set(proj TutorialStep1-LocalNoDirTAR)
 add_external_project(${proj}
   TAR "${CMAKE_CURRENT_SOURCE_DIR}/Step1NoDir.tar"
+  LIST_SEPARATOR @@
   CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=${prefix} -G ${CMAKE_GENERATOR} ${source_dir}/${proj}
+             -DTEST_LIST:STRING=1@@2@@3
   INSTALL_COMMAND ""
 )
 add_external_project_step(${proj} mypatch

Tests/ExternalProject/Step1Patch.cmake

@@ -17,5 +17,9 @@ endif()
 file(APPEND CMakeLists.txt "
 # Patch by ExternalProject test:
 set_property(TARGET Tutorial PROPERTY OUTPUT_NAME EP-Tutorial)
+list(LENGTH TEST_LIST len)
+if(NOT len EQUAL 3)
+  message(FATAL_ERROR \"TEST_LIST length is \${len}, not 3\")
+endif()
 ")
 message(STATUS "Patched ${CMAKE_CURRENT_SOURCE_DIR}/CMakeLists.txt")