Catch2's tests should never be run, if it is loaded as a sub-project

[claremacrae]

Please note: I will submit PRs to fix this on the v2 and v3 branches... the fix is trivial

Describe the bug

Catch2's top-level CMakeLists.txt file has code to prevent self-tests being run if the project is being bundled e.g. via add_subdirectory() or FetchContent.

# detect if Catch is being bundled,
# disable testsuite in that case
if(NOT DEFINED PROJECT_NAME)
    set(NOT_SUBPROJECT ON)
endif()

However, if NOT_SUBPROJECT was already set to ON in a parent scope, the self-tests are still run anyway (and also the 20-30 CTest targets are added)

Expected behavior

Turn off NOT_SUBPROJECT if Catch2 is running as a sub-project

Reproduction steps

Steps to reproduce the bug.

git clone https://github.com/claremacrae/demo-cmake-subproject-error.git

mkdir build
cd    build

cmake ..
cmake --build . --target help

Expected output:

The following are some of the valid targets for this Makefile:
... all (the default if no target is provided)
... clean
... depend
... rebuild_cache
... edit_cache
... package_source
... package
... Catch2WithMain
... tests

Actual output - note the include of SelfTest, as well as all the targets generated by including CTest:

The following are some of the valid targets for this Makefile:
... all (the default if no target is provided)
... clean
... depend
... install/local
... install/strip
... install
... package_source
... rebuild_cache
... edit_cache
... list_install_components
... package
... Catch2WithMain
... ContinuousCoverage
... ContinuousTest
... ContinuousBuild
... ContinuousStart
... ExperimentalBuild
... ExperimentalCoverage
... ExperimentalSubmit
... ContinuousUpdate
... ExperimentalUpdate
... ExperimentalStart
... ContinuousSubmit
... ExperimentalTest
... NightlySubmit
... NightlyMemCheck
... ExperimentalConfigure
... NightlyCoverage
... NightlyTest
... ContinuousMemCheck
... NightlyBuild
... NightlyConfigure
... NightlyUpdate
... NightlyMemoryCheck
... ContinuousConfigure
... Nightly
... NightlyStart
... ExperimentalMemCheck
... Continuous
... Experimental
... SelfTest
... tests

Platform information:

• OS: Windows NT
• Compiler+version: GCC v2.9.5
• Catch version: v1.2.3

Additional context
Add any other context about the problem here.

[claremacrae]

The fix is trivial:

# detect if Catch is being bundled,
# disable testsuite in that case
if(NOT DEFINED PROJECT_NAME)
    set(NOT_SUBPROJECT ON)
+else()
+    set(NOT_SUBPROJECT OFF)
endif()

[claremacrae]

The with-catch2-v3 branch of claremacrae/demo-cmake-subproject-error allows the behaviour to be tested against Catch2's v3 code, i.e. the devel branch.