Cleanup and use BEMAN_HAS_MODULES in code

[ClausKlein]

Create and install an interface libray and the CXX_MODULE if possible

Available CMake options:

BEMAN_EXECUTION_BUILD_EXAMPLES:BOOL=ON
BEMAN_EXECUTION_ENABLE_INSTALL:BOOL=ON
BEMAN_EXECUTION_ENABLE_TESTING:BOOL=ON
BEMAN_HAS_IMPORT_STD:BOOL=ON
# -> "Build with import std; is possible and used!"

if(CMAKE_CXX_STANDARD GREATER_EQUAL 20)
    option(BEMAN_USE_MODULES "Build CXX_MODULES" ${CMAKE_CXX_SCAN_FOR_MODULES})
endif()
if(BEMAN_USE_MODULES)
    target_compile_definitions(beman_execution PUBLIC BEMAN_HAS_MODULES)
endif()

BEMAN_USE_STD_MODULE:BOOL=ON
# -> "Check if 'import std;' is possible with the toolchain?"

if(BEMAN_USE_MODULES AND CMAKE_CXX_MODULE_STD)
    target_compile_definitions(beman_execution PUBLIC BEMAN_HAS_IMPORT_STD)
endif()

Exported CMake targets:

beman_execution imported as beman::beman_execution
beman_execution_headers imported as beman::beman_execution_headers

Installed CMAKE config package:

bash-5.3$ cpack
CPack: Create package using TGZ
CPack: Install projects
CPack: - Install project: beman_execution []
CPack: Create package
CPack: - package: gcc-release/beman_execution-0.0.1-Darwin.tar.gz generated.
bash-5.3$ tree _CPack_Packages/Darwin/TGZ/beman_execution-0.0.1-Darwin/lib/cmake/
_CPack_Packages/Darwin/TGZ/beman_execution-0.0.1-Darwin/lib/cmake/
└── beman_execution
    ├── beman_execution-config-targets-relwithdebinfo.cmake
    ├── beman_execution-config-targets.cmake
    ├── beman_execution-config-version.cmake
    ├── beman_execution-config.cmake
    ├── cxx-modules-beman_execution-config-targets-RelWithDebInfo.cmake
    ├── cxx-modules-beman_execution-config-targets.cmake
    ├── module
    │   └── execution.cppm
    └── target-beman__execution-RelWithDebInfo.cmake

3 directories, 8 files
bash-5.3$

[ClausKlein]

@dietmarkuehl ninja examples/all and and ninja all_verify_interface_header_sets build fine

but some tests fails to build with import beman.execution;!

only this tests fail to compile:

tests/beman/execution/exec-affine-on.test.cpp
tests/beman/execution/issue-174.test.cpp
tests/beman/execution/issue-186.test.cpp
tests/beman/execution/exec-scope-counting.test.cpp
tests/beman/execution/exec-prop.test.cpp
tests/beman/execution/exec-env.test.cpp
tests/beman/execution/exec-scope-simple-counting.test.cpp
tests/beman/execution/exec-scope-concepts.test.cpp
tests/beman/execution/issue-144.test.cpp
tests/beman/execution/exec-awaitable.test.cpp
tests/beman/execution/exec-fwd-env.test.cpp
tests/beman/execution/exec-general.test.cpp
tests/beman/execution/exec-get-compl-sched.test.cpp
tests/beman/execution/exec-get-delegation-scheduler.test.cpp
tests/beman/execution/exec-get-domain.test.cpp
tests/beman/execution/exec-get-env.test.cpp
tests/beman/execution/exec-get-scheduler.test.cpp
tests/beman/execution/exec-get-stop-token.test.cpp
tests/beman/execution/exec-getcomplsigs.test.cpp
tests/beman/execution/exec-into-variant.test.cpp
tests/beman/execution/exec-let.test.cpp
tests/beman/execution/exec-opstate-start.test.cpp
tests/beman/execution/exec-opstate.test.cpp
tests/beman/execution/exec-read-env.test.cpp
tests/beman/execution/exec-run-loop-general.test.cpp
tests/beman/execution/exec-run-loop-types.test.cpp
tests/beman/execution/exec-sched.test.cpp
tests/beman/execution/exec-schedule-from.test.cpp
tests/beman/execution/exec-schedule.test.cpp
tests/beman/execution/exec-set-error.test.cpp
tests/beman/execution/exec-set-stopped.test.cpp
tests/beman/execution/exec-set-value.test.cpp
tests/beman/execution/exec-snd-apply.test.cpp
tests/beman/execution/exec-snd-transform.test.cpp
tests/beman/execution/exec-starts-on.test.cpp
tests/beman/execution/exec-sync-wait.test.cpp
tests/beman/execution/exec-then.test.cpp
tests/beman/execution/exec-when-all.test.cpp
tests/beman/execution/exec-with-awaitable-senders.test.cpp
tests/beman/execution/exec-bulk.test.cpp
tests/beman/execution/function-objects.test.cpp
tests/beman/execution/stopcallback-cons.test.cpp
tests/beman/execution/stopcallback-general.test.cpp
tests/beman/execution/stopcallback-inplace-cons.test.cpp
tests/beman/execution/stopcallback-inplace-general.test.cpp
tests/beman/execution/stopcallback-inplace.test.cpp
tests/beman/execution/stopcallback.test.cpp
tests/beman/execution/stopsource-cons.test.cpp
tests/beman/execution/stopsource-general.test.cpp
tests/beman/execution/stopsource-inplace-cons.test.cpp
tests/beman/execution/stopsource-inplace-general.test.cpp
tests/beman/execution/stopsource-inplace-mem.test.cpp
tests/beman/execution/stopsource-mem.test.cpp
tests/beman/execution/stoptoken-general.test.cpp
tests/beman/execution/stoptoken-inplace-general.test.cpp
tests/beman/execution/stoptoken-inplace-members.test.cpp
tests/beman/execution/stoptoken-mem.test.cpp
tests/beman/execution/stoptoken-never-general.test.cpp
tests/beman/execution/thread-stoptoken-intro.test.cpp

[ClausKlein]

@dietmarkuehl I may comment this out for now?

[dietmarkuehl]

I'll merge the PR onto the branch about nowish. ... and I can comment that line out if that is all what's needed. On the other hand, I want to make sure I test module building when meddling with the script.