cmLocalCommonGenerator: Save CMAKE_BUILD_TYPE on construction

Copy the value to our `ConfigName` member on construction to ensure it
is available even to code paths that run before `Generate`.  We once
needed to delay this lookup until `Generate` because the local
generators were at one time created before `Configure`.  Now they are
created at generate time which is late enough to expect
`CMAKE_BUILD_TYPE` to be available.

Without this, `cmGlobalUnixMakefileGenerator3::WriteConvenienceRules`
causes use of `ConfigName` before it is populated which breaks use of
source files that depend on the `$<CONFIG>` generator expression.

Fixes: #16889

Source/cmLocalCommonGenerator.cxx

@@ -15,14 +15,6 @@ cmLocalCommonGenerator::cmLocalCommonGenerator(cmGlobalGenerator* gg,
                                                std::string const& wd)
   : cmLocalGenerator(gg, mf)
   , WorkingDirectory(wd)
-{
-}
-
-cmLocalCommonGenerator::~cmLocalCommonGenerator()
-{
-}
-
-void cmLocalCommonGenerator::SetConfigName()
 {
   // Store the configuration name that will be generated.
   if (const char* config = this->Makefile->GetDefinition("CMAKE_BUILD_TYPE")) {
@@ -34,6 +26,10 @@ void cmLocalCommonGenerator::SetConfigName()
   }
 }
 
+cmLocalCommonGenerator::~cmLocalCommonGenerator()
+{
+}
+
 std::string cmLocalCommonGenerator::GetTargetFortranFlags(
   cmGeneratorTarget const* target, std::string const& config)
 {

Source/cmLocalCommonGenerator.h

@@ -33,7 +33,6 @@ public:
 protected:
   std::string WorkingDirectory;
 
-  void SetConfigName();
   std::string ConfigName;
 
   friend class cmCommonTargetGenerator;

Source/cmLocalNinjaGenerator.cxx

@@ -58,8 +58,6 @@ void cmLocalNinjaGenerator::Generate()
     this->HomeRelativeOutputPath = "";
   }
 
-  this->SetConfigName();
-
   this->WriteProcessedMakefile(this->GetBuildFileStream());
 #ifdef NINJA_GEN_VERBOSE_FILES
   this->WriteProcessedMakefile(this->GetRulesFileStream());

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -105,8 +105,6 @@ cmLocalUnixMakefileGenerator3::~cmLocalUnixMakefileGenerator3()
 
 void cmLocalUnixMakefileGenerator3::Generate()
 {
-  this->SetConfigName();
-
   // Record whether some options are enabled to avoid checking many
   // times later.
   if (!this->GetGlobalGenerator()->GetCMakeInstance()->GetIsInTryCompile()) {