find_package: Improve CMP0074 warning messages

Policy `CMP0074` was added by commit eb35d8884b (find_package: Use
PackageName_ROOT variables as search prefixes, 2018-03-15).  Revise the
logic to avoid warning when a `PackageName_ROOT` variable is set to
empty since that won't change the search behavior.  Also, when we do
warn include the variable value(s) for reference.

1 files changed, 10 insertions, 4 deletions

diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index 1191490..5850f94 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -162,16 +162,22 @@ void cmMakefile::MaybeWarnCMP0074(std::string const& pkg)
 {
   // Warn if a <pkg>_ROOT variable we may use is set.
   std::string const varName = pkg + "_ROOT";
-  bool const haveVar = this->GetDefinition(varName) != nullptr;
-  bool const haveEnv = cmSystemTools::HasEnv(varName);
+  const char* var = this->GetDefinition(varName);
+  std::string env;
+  cmSystemTools::GetEnv(varName, env);
+
+  bool const haveVar = var && *var;
+  bool const haveEnv = !env.empty();
   if ((haveVar || haveEnv) && this->WarnedCMP0074.insert(varName).second) {
     std::ostringstream w;
     w << cmPolicies::GetPolicyWarning(cmPolicies::CMP0074) << "\n";
     if (haveVar) {
-      w << "CMake variable " << varName << " is set.\n";
+      w << "CMake variable " << varName << " is set to:\n"
+        << "  " << var << "\n";
     }
     if (haveEnv) {
-      w << "Environment variable " << varName << " is set.\n";
+      w << "Environment variable " << varName << " is set to:\n"
+        << "  " << env << "\n";
     }
     w << "For compatibility, CMake is ignoring the variable.";
     this->IssueMessage(cmake::AUTHOR_WARNING, w.str());