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, 3 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index 04e9dc1..abbc001 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -70,6 +70,9 @@ public:
   /** Tell the generator about the target system.  */
   virtual bool SetSystemName(std::string const&, cmMakefile*) { return true; }
 
+  /** Set the generator-specific instance.  Returns true if supported.  */
+  virtual bool SetGeneratorInstance(std::string const& i, cmMakefile* mf);
+
   /** Set the generator-specific platform name.  Returns true if platform
       is supported and false otherwise.  */
   virtual bool SetGeneratorPlatform(std::string const& p, cmMakefile* mf);