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.

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());

Tests/RunCMake/find_package/CMP0074-WARN-stderr.txt

@@ -7,9 +7,13 @@ CMake Warning \(dev\) at CMP0074-common.cmake:[0-9]+ \(find_package\):
   Run "cmake --help-policy CMP0074" for policy details.  Use the cmake_policy
   command to set the policy and suppress this warning.
 
-  CMake variable Foo_ROOT is set.
+  CMake variable Foo_ROOT is set to:
 
-  Environment variable Foo_ROOT is set.
+    .*/Tests/RunCMake/find_package/PackageRoot/foo/cmake_root
+
+  Environment variable Foo_ROOT is set to:
+
+    .*/Tests/RunCMake/find_package/PackageRoot/foo/env_root
 
   For compatibility, CMake is ignoring the variable.
 Call Stack \(most recent call first\):