cmMakefile: Improve ExpandVariablesInString return type

Return `std::string const&` instead of a `const char*` that points
into a string anyway.  Update call sites accordingly.

Source/cmCPluginAPI.cxx

@@ -405,7 +405,8 @@ char CCONV* cmExpandVariablesInString(void* arg, const char* source,
 {
   cmMakefile* mf = static_cast<cmMakefile*>(arg);
   std::string barf = source;
-  std::string result = mf->ExpandVariablesInString(barf, escapeQuotes, atOnly);
+  std::string const& result =
+    mf->ExpandVariablesInString(barf, escapeQuotes, atOnly);
   return strdup(result.c_str());
 }
 

Source/cmMakefile.cxx

@@ -2412,12 +2412,13 @@ std::vector<std::string> cmMakefile::GetDefinitions() const
   return res;
 }
 
-const char* cmMakefile::ExpandVariablesInString(std::string& source) const
+const std::string& cmMakefile::ExpandVariablesInString(
+  std::string& source) const
 {
   return this->ExpandVariablesInString(source, false, false);
 }
 
-const char* cmMakefile::ExpandVariablesInString(
+const std::string& cmMakefile::ExpandVariablesInString(
   std::string& source, bool escapeQuotes, bool noEscapes, bool atOnly,
   const char* filename, long line, bool removeEmpty, bool replaceAt) const
 {
@@ -2433,7 +2434,7 @@ const char* cmMakefile::ExpandVariablesInString(
     this->IssueMessage(cmake::INTERNAL_ERROR,
                        "ExpandVariablesInString @ONLY called "
                        "on something with escapes.");
-    return source.c_str();
+    return source;
   }
 
   // Variables used in the WARN case.
@@ -2515,7 +2516,7 @@ const char* cmMakefile::ExpandVariablesInString(
     this->IssueMessage(cmake::AUTHOR_WARNING, msg);
   }
 
-  return source.c_str();
+  return source;
 }
 
 cmake::MessageType cmMakefile::ExpandVariablesInStringOld(

Source/cmMakefile.h

@@ -565,12 +565,11 @@ public:
    * entry in the this->Definitions map.  Also \@var\@ is
    * expanded to match autoconf style expansions.
    */
-  const char* ExpandVariablesInString(std::string& source) const;
-  const char* ExpandVariablesInString(std::string& source, bool escapeQuotes,
-                                      bool noEscapes, bool atOnly = false,
-                                      const char* filename = nullptr,
-                                      long line = -1, bool removeEmpty = false,
-                                      bool replaceAt = false) const;
+  const std::string& ExpandVariablesInString(std::string& source) const;
+  const std::string& ExpandVariablesInString(
+    std::string& source, bool escapeQuotes, bool noEscapes,
+    bool atOnly = false, const char* filename = nullptr, long line = -1,
+    bool removeEmpty = false, bool replaceAt = false) const;
 
   /**
    * Remove any remaining variables in the string. Anything with ${var} or