First step to install cxx_module

[ClausKlein]

• works with clang-19 only on Linux and macOS
• msvc makes trouble as always

D:\a\execution26\execution26\src\beman\execution26\execution.cppm(223): fatal error C1001: Internal compiler error.
(compiler file 'D:\a\_work\1\s\src\vctools\Compiler\CxxFE\sl\p1\c\module\writer.cpp', line 6916)
 To work around this problem, try simplifying or changing the program near the locations listed above.
If possible please provide a repro here: https://developercommunity.visualstudio.com/ 
Please choose the Technical Support command on the Visual C++ 
 Help menu, or open the Technical Support help file for more information

fixes issue #93

[ClausKlein]

Now it install the following cmake config packages for Debug and RelWithDebInfo:

tree stagedir/lib/
stagedir/lib/
|-- cmake
|   `-- beman_execution26
|       |-- beman_execution26-config-version.cmake
|       |-- beman_execution26-config.cmake
|       |-- beman_execution26-targets-debug.cmake
|       |-- beman_execution26-targets.cmake
|       |-- cxx-modules-beman_execution26-targets-Debug.cmake
|       |-- cxx-modules-beman_execution26-targets-RelWithDebInfo.cmake
|       |-- cxx-modules-beman_execution26-targets.cmake
|       |-- src
|       |   `-- execution.cppm
|       |-- target-beman__execution26-Debug.cmake
|       `-- target-beman__execution26-RelWithDebInfo.cmake
|-- libbeman_execution26.a
`-- libbeman_execution26D.a

4 directories, 12 files

@neatudarius what do you think about that?

[dietmarkuehl]

The changes look good. The only bit which I'm a bit concerned about is disabling g++ on Linux.

[dietmarkuehl]

g++ is the default compiler on Linuxes. I understand that modules are only [properly] implemented with g++-15 but it seems unfortunate to remove the default compiler entirely: is it possible to setup cmake to use modules only for some compilers?

[ClausKlein]

This should be possible, fmt, boost do so

Is g++15 available to install on ubuntu 24.04?
If so, we should try it.

On macOS, I have only g++14 which fail to compile the module code.

[dietmarkuehl]

I had problems without this definition. Adding it resulted in things building on MacOS. Probably this part of the code can be removed.

[ClausKlein]

yes