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

[jcfr]

This ensures the project and its dependency are built using the expected compiler.

With this set of changes, the extension project now build on the Slicer build machine where we explicitly set CC and CXX variables to /dev/null and ensure the same compiler is used across all projects.

It fixes the following error:

-- Extension description has been written to: /.../DiffusionQC-build/DiffusionQC.s4ext
-- Checking if extension type is SuperBuild
-- Checking if extension type is SuperBuild - true
-- Extension fixup mode: updating CPACK_INSTALL_CMAKE_PROJECTS with <cpack_bundle_fixup_directory>
-- The C compiler identification is unknown
-- The CXX compiler identification is unknown
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working C compiler: /dev/null
-- Check for working C compiler: /dev/null - broken
...

Source: https://slicer.cdash.org/viewBuildError.php?buildid=3001519

Summary:

• 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.

If these changes, the extension now build successfully:

References:

• computron: Attempt to fix clang segfault updating from 14.0.0 to 14.0.6 Slicer/DashboardScripts#57
• https://discourse.slicer.org/t/transition-of-macos-preview-build-from-host-10-13-high-sierra-to-13-ventura/28668/3?u=jcfr

cc: @sjh26 @Sunderlandkyl