COMP: Fix build ensuring inner-build doesn't override the outer build

* Ensure inner-build doesn't override the outer build by setting
  the EXTENSION_BUILD_SUBDIRECTORY variable specific to the SuperBuild
  extension layout.

* Fix inner build ensuring the Slicer_DIR variable is propagated.

* Ensure the Git executable is consistently set and remove obsolete
  workaround. Note also that Subversion is not used anymore.

* Ensure CMake options specific to CXX standard are propagated.

* Remove obsolete <proj>_USE_GIT_PROTOCOL variable. It is now defined and
  set in the ExternalProjectDependency CMake module.
  Note also that the default is now set to "https" as GitHub removed
  unauthenticated access through the git protocol.
  See https://github.blog/2021-09-01-improving-git-protocol-security-github/

* Remove obsolete MIDAS_PACKAGE_* variables. Upload of extension is done
  using a different mechanism. See Slicer/Slicer@359f95c9b (COMP: Simplify
  extension package upload logic removing midas support)

* Remove explicit and incorrect setting of CMAKE_BUILD_TYPE option. The option
  is set only for single-config CMake generator (e.g Unix Makefiles) after
  calling "include(${Slicer_USE_FILE})" which indirectly includes the
  "SlicerInitializeBuildType" CMake module.

CMakeLists.txt

@@ -11,11 +11,16 @@ set(EXTENSION_DESCRIPTION "A Slicer module for automated and interactive quality
 set(EXTENSION_ICONURL "https://raw.githubusercontent.com/pnlbwh/SlicerDiffusionQC/master/Misc/DiffusionQC-icon-128x128.png")
 set(EXTENSION_SCREENSHOTURLS "https://github.com/pnlbwh/SlicerDiffusionQC/raw/master/Misc/DiffusionQC-screenshot.jpg")
 set(EXTENSION_DEPENDS "SlicerDMRI") # Specified as a space separated string, a list or 'NA' if any
+set(EXTENSION_BUILD_SUBDIRECTORY inner-build)
 
 #-----------------------------------------------------------------------------
 # Extension dependencies
 find_package(Slicer REQUIRED)
 include(${Slicer_USE_FILE})
+mark_as_superbuild(Slicer_DIR)
+
+find_package(Git REQUIRED)
+mark_as_superbuild(GIT_EXECUTABLE)
 
 #-----------------------------------------------------------------------------
 # SuperBuild setup

SuperBuild.cmake

@@ -1,16 +1,6 @@
 include(SlicerCheckCMakeHTTPS)
 slicer_check_cmake_https()
 
-#-----------------------------------------------------------------------------
-# Git protocol option
-#-----------------------------------------------------------------------------
-option(${CMAKE_PROJECT_NAME}_USE_GIT_PROTOCOL "If behind a firewall turn this off to use http instead." ON)
-
-set(git_protocol "git")
-if(NOT ${CMAKE_PROJECT_NAME}_USE_GIT_PROTOCOL)
-  set(git_protocol "http")
-endif()
-
 #-----------------------------------------------------------------------------
 # Enable and setup External project global properties
 #-----------------------------------------------------------------------------
@@ -36,14 +26,6 @@ ExternalProject_Include_Dependencies(${proj}
   SUPERBUILD_VAR ${EXTENSION_NAME}_SUPERBUILD
   )
 
-# XXX Workaround https://gitlab.kitware.com/cmake/cmake/issues/15448
-#     and explicitly pass GIT_EXECUTABLE and Subversion_SVN_EXECUTABLE
-foreach(varname IN ITEMS GIT_EXECUTABLE Subversion_SVN_EXECUTABLE)
-  if(EXISTS "${${varname}}")
-    list(APPEND DCMQI_EP_CMAKE_CACHE_ARGS -D${varname}:FILEPATH=${${varname}})
-  endif()
-endforeach()
-
 ExternalProject_Add(${proj}
   ${${proj}_EP_ARGS}
   DOWNLOAD_COMMAND ""
@@ -52,19 +34,21 @@ ExternalProject_Add(${proj}
   BINARY_DIR ${EXTENSION_BUILD_SUBDIRECTORY}
   BUILD_ALWAYS 1
   CMAKE_CACHE_ARGS
+    # Compiler settings
     -DCMAKE_CXX_COMPILER:FILEPATH=${CMAKE_CXX_COMPILER}
     -DCMAKE_CXX_FLAGS:STRING=${ep_common_cxx_flags}
     -DCMAKE_C_COMPILER:FILEPATH=${CMAKE_C_COMPILER}
     -DCMAKE_C_FLAGS:STRING=${ep_common_c_flags}
-    -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
+    -DCMAKE_CXX_STANDARD:STRING=${CMAKE_CXX_STANDARD}
+    -DCMAKE_CXX_STANDARD_REQUIRED:BOOL=${CMAKE_CXX_STANDARD_REQUIRED}
+    -DCMAKE_CXX_EXTENSIONS:BOOL=${CMAKE_CXX_EXTENSIONS}
+    # Output directories
     -DCMAKE_RUNTIME_OUTPUT_DIRECTORY:PATH=${CMAKE_RUNTIME_OUTPUT_DIRECTORY}
     -DCMAKE_LIBRARY_OUTPUT_DIRECTORY:PATH=${CMAKE_LIBRARY_OUTPUT_DIRECTORY}
     -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY:PATH=${CMAKE_ARCHIVE_OUTPUT_DIRECTORY}
-    -DMIDAS_PACKAGE_EMAIL:STRING=${MIDAS_PACKAGE_EMAIL}
-    -DMIDAS_PACKAGE_API_KEY:STRING=${MIDAS_PACKAGE_API_KEY}
+    # Superbuild
     -D${EXTENSION_NAME}_SUPERBUILD:BOOL=OFF
     -DEXTENSION_SUPERBUILD_BINARY_DIR:PATH=${${EXTENSION_NAME}_BINARY_DIR}
-    ${DCMQI_EP_CMAKE_CACHE_ARGS}
   DEPENDS
     ${${proj}_DEPENDS}
   )