Add infrastructure for generators to select a build tool instance

Add cache entry `CMAKE_GENERATOR_INSTANCE` to hold the instance location
persistently across re-runs of CMake in a given build tree.

For now we reject the option by default if explicitly set.  It will be
implemented on a per-generator basis.  Pass the setting into try_compile
project generation.  Add a RunCMake.GeneratorInstance test to cover
basic use cases for the option.  Verify that `CMAKE_GENERATOR_INSTANCE`
is empty by default, and that it is rejected when the generator does not
support a user setting.

Issue: #17268

1 files changed, 6 insertions, 0 deletions

diff --git a/Tests/RunCMake/RunCMake.cmake b/Tests/RunCMake/RunCMake.cmake
index 4fd816e..e688830 100644
--- a/Tests/RunCMake/RunCMake.cmake
+++ b/Tests/RunCMake/RunCMake.cmake
@@ -79,11 +79,17 @@ function(run_cmake test)
       ${maybe_timeout}
       )
   else()
+    if(RunCMake_GENERATOR_INSTANCE)
+      set(_D_CMAKE_GENERATOR_INSTANCE "-DCMAKE_GENERATOR_INSTANCE=${RunCMake_GENERATOR_INSTANCE}")
+    else()
+      set(_D_CMAKE_GENERATOR_INSTANCE "")
+    endif()
     execute_process(
       COMMAND ${CMAKE_COMMAND} "${RunCMake_TEST_SOURCE_DIR}"
                 -G "${RunCMake_GENERATOR}"
                 -A "${RunCMake_GENERATOR_PLATFORM}"
                 -T "${RunCMake_GENERATOR_TOOLSET}"
+                ${_D_CMAKE_GENERATOR_INSTANCE}
                 -DRunCMake_TEST=${test}
                 --no-warn-unused-cli
                 ${RunCMake_TEST_OPTIONS}