Gate CMake tests behind AU_ENABLE_TESTING option (#303)

This brings in the external contributions from #297, merges in the
latest main, and prepends an `AU_` prefix to the option.

A future PR will add another option to let users opt out of the GTest
dependency entirely, losing access to the `Au::testing` target (which
wouldn't be any loss to people who don't use GTest).

To test:

```sh
cmake \
  -S . \
  -B cmake/build \
  -DAU_ENABLE_TESTING=FALSE \
  -DCMAKE_VERIFY_INTERFACE_HEADER_SETS=TRUE \
&& cmake \
  --build cmake/build \
  --target all
```

Helps #257.

---------

Co-authored-by: Riff, Eric <[email protected]>

CMakeLists.txt

@@ -32,10 +32,17 @@ project(
    LANGUAGES CXX
 )
 
+option(AU_ENABLE_TESTING "Build Unit Tests" ON)
+
 # The export set for all of our headers.
 set(AU_EXPORT_SET_NAME AuHeaders)
 
-enable_testing()
+if(AU_ENABLE_TESTING)
+   message(STATUS "Unit tests enabled")
+   enable_testing()
+else()
+   message(STATUS "Unit tests disabled")
+endif()
 
 # Bring in GoogleTest so we can build and run the tests.
 include(FetchContent)
@@ -55,6 +62,8 @@ set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 FetchContent_MakeAvailable(googletest)
 include(GoogleTest)
 
+set(AU_CONFIG_FIND_GTEST_LINE "find_dependency(googletest 1.12.1)")
+
 add_subdirectory(au)
 
 # Configure how Au will be installed.

cmake/AuConfig.cmake.in

@@ -15,7 +15,7 @@
 @PACKAGE_INIT@
 
 include(CMakeFindDependencyMacro)
-find_dependency(googletest 1.12.1)
+@AU_CONFIG_FIND_GTEST_LINE@
 
 include(${CMAKE_CURRENT_LIST_DIR}/AuHeaders.cmake)
 

cmake/HeaderOnlyLibrary.cmake

@@ -74,6 +74,11 @@ function(header_only_library)
 endfunction()
 
 function(gtest_based_test)
+  if (NOT AU_ENABLE_TESTING)
+    message(VERBOSE "AU_ENABLE_TESTING not defined; will not create test target.")
+    return()
+  endif()
+
   #
   # Handle argument parsing
   #