Changes to MPD necessary to support Cetmodules >=4

[greenc-FNAL]

In order to support development of packages using Cetmodules 4 when released, the following additions will need to be made to MPD functionality:

• The generated presets file should include the clause in a "cacheVariables" table:

  "CMAKE_PROJECT_TOP_LEVEL_INCLUDES" : {
     "type" : "STRING",
     "value" : "CetProvideDependency"
  }

• The top-level CMakeLists.txt should contain the following code

  ########################################################################
  # Ensure we have access to a suitable version of Cetmodules
  include(FetchContent)
  FetchContent_Declare(
    cetmodules
    GIT_REPOSITORY https://github.com/FNALssi/cetmodules
    GIT_TAG 4.00.00
    FIND_PACKAGE_ARGS 4.00.00
  )
  FetchContent_MakeAvailable(cetmodules)
  ########################################################################
  
  find_package(cetmodules 4.00.00 REQUIRED)

  immediately before the project() call. Following best practice, GIT_TAG should actually be the commit hash to which the desired tag points.

N.B. If CMAKE_PREFIX_PATH points to a suitable version of Cetmodules matching FIND_PACKAGE_ARGS, then it will be used instead of downloading the source via FetchContent.

Including Cetmodules at this stage should not prevent the development of packages that do not use Cetmodules themselves.

[greenc-FNAL]

Incidentally, this should resolve (or at least enable the straightforward resolution of) #10.

[knoepfel]

@greenc-FNAL, have you pushed the Cetmodules 4.0.0 tag?

==> Building with command:

cmake --build /scratch/knoepfel/meld-devel/build -- -j12

[0/1] Re-running CMake...
[1/9] Creating directories for 'cetmodules-populate'
[1/9] Performing download step (git clone) for 'cetmodules-populate'
Cloning into 'cetmodules-src'...
fatal: invalid reference: 4.00.00
CMake Error at cetmodules-subbuild/cetmodules-populate-prefix/tmp/cetmodules-populate-gitclone.cmake:61 (message):
  Failed to checkout tag: '4.00.00'

[greenc-FNAL]

No, sorry. Still working on that.