try_compile: Add policy CMP0066 to honor CMAKE_<LANG>_FLAGS_<CONFIG>

In the `try_compile` source file signature we propagate the caller's
value of `CMAKE_<LANG>_FLAGS` into the test project.  Extend this to
propagate `CMAKE_<LANG>_FLAGS_<CONFIG>` too instead of always using the
default value in the test project.  This will be useful, for example, to
allow the MSVC runtime library to be changed (e.g. `-MDd` => `-MTd`).
However, some projects may currently depend on this not being done,
so we need to activate the behavior using a policy.

This change was originally made by commit v3.6.0-rc1~160^2 (try_compile:
Honor CMAKE_<LANG>_FLAGS_<CONFIG> changes, 2016-04-11) but without the
policy and so had to be reverted during the 3.6 release candidate cycle.

Fixes #16174.

1 files changed, 37 insertions, 0 deletions

diff --git a/Source/cmCoreTryCompile.cxx b/Source/cmCoreTryCompile.cxx
index 805d8e8..e9367b1 100644
--- a/Source/cmCoreTryCompile.cxx
+++ b/Source/cmCoreTryCompile.cxx
@@ -334,6 +334,43 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
                     " ${COMPILE_DEFINITIONS}\")\n",
               li->c_str(), li->c_str());
     }
+    switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0066)) {
+      case cmPolicies::WARN:
+        if (this->Makefile->PolicyOptionalWarningEnabled(
+              "CMAKE_POLICY_WARNING_CMP0066")) {
+          std::ostringstream w;
+          /* clang-format off */
+          w << cmPolicies::GetPolicyWarning(cmPolicies::CMP0066) << "\n"
+            "For compatibility with older versions of CMake, try_compile "
+            "is not honoring caller config-specific compiler flags "
+            "(e.g. CMAKE_C_FLAGS_DEBUG) in the test project."
+            ;
+          /* clang-format on */
+          this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, w.str());
+        }
+      case cmPolicies::OLD:
+        // OLD behavior is to do nothing.
+        break;
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS:
+        this->Makefile->IssueMessage(
+          cmake::FATAL_ERROR,
+          cmPolicies::GetRequiredPolicyError(cmPolicies::CMP0066));
+      case cmPolicies::NEW: {
+        // NEW behavior is to pass config-specific compiler flags.
+        static std::string const cfgDefault = "DEBUG";
+        std::string const cfg =
+          !tcConfig.empty() ? cmSystemTools::UpperCase(tcConfig) : cfgDefault;
+        for (std::set<std::string>::iterator li = testLangs.begin();
+             li != testLangs.end(); ++li) {
+          std::string const langFlagsCfg = "CMAKE_" + *li + "_FLAGS_" + cfg;
+          const char* flagsCfg = this->Makefile->GetDefinition(langFlagsCfg);
+          fprintf(fout, "set(%s %s)\n", langFlagsCfg.c_str(),
+                  cmOutputConverter::EscapeForCMake(flagsCfg ? flagsCfg : "")
+                    .c_str());
+        }
+      } break;
+    }
     switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0056)) {
       case cmPolicies::WARN:
         if (this->Makefile->PolicyOptionalWarningEnabled(