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, 7 insertions, 0 deletions

diff --git a/Source/cmake.h b/Source/cmake.h
index ed3ebe0..5c5a90d 100644
--- a/Source/cmake.h
+++ b/Source/cmake.h
@@ -203,6 +203,12 @@ public:
   ///! Get the names of the current registered generators
   void GetRegisteredGenerators(std::vector<GeneratorInfo>& generators) const;
 
+  ///! Set the name of the selected generator-specific instance.
+  void SetGeneratorInstance(std::string const& instance)
+  {
+    this->GeneratorInstance = instance;
+  }
+
   ///! Set the name of the selected generator-specific platform.
   void SetGeneratorPlatform(std::string const& ts)
   {
@@ -431,6 +437,7 @@ protected:
 
   cmGlobalGenerator* GlobalGenerator;
   std::map<std::string, DiagLevel> DiagLevels;
+  std::string GeneratorInstance;
   std::string GeneratorPlatform;
   std::string GeneratorToolset;