cmExecuteProcessCommand: Explicitly ignore unknown ENCODING values

The logic did this implicitly before.  Make it easier to follow.

Source/cmExecuteProcessCommand.cxx

@@ -11,6 +11,7 @@
 #include <utility>
 #include <vector>
 
+#include <cm/optional>
 #include <cm/string_view>
 #include <cmext/algorithm>
 #include <cmext/string_view>
@@ -69,7 +70,7 @@ bool cmExecuteProcessCommand(std::vector<std::string> const& args,
     bool ErrorStripTrailingWhitespace = false;
     bool EchoOutputVariable = false;
     bool EchoErrorVariable = false;
-    std::string Encoding;
+    cm::optional<std::string> Encoding;
     std::string CommandErrorIsFatal;
   };
 
@@ -296,8 +297,14 @@ bool cmExecuteProcessCommand(std::vector<std::string> const& args,
   };
   ReadData outputData;
   ReadData errorData;
-  cmProcessOutput processOutput(
-    cmProcessOutput::FindEncoding(arguments.Encoding));
+  cmProcessOutput::Encoding encoding = cmProcessOutput::Auto;
+  if (arguments.Encoding) {
+    if (cm::optional<cmProcessOutput::Encoding> maybeEncoding =
+          cmProcessOutput::FindEncoding(*arguments.Encoding)) {
+      encoding = *maybeEncoding;
+    }
+  }
+  cmProcessOutput processOutput(encoding);
   std::string strdata;
 
   std::unique_ptr<cmUVStreamReadHandle> outputHandle;

Source/cmProcessOutput.cxx

@@ -12,16 +12,18 @@ unsigned int cmProcessOutput::defaultCodepage =
   KWSYS_ENCODING_DEFAULT_CODEPAGE;
 #endif
 
-cmProcessOutput::Encoding cmProcessOutput::FindEncoding(
+cm::optional<cmProcessOutput::Encoding> cmProcessOutput::FindEncoding(
   std::string const& name)
 {
-  Encoding encoding = Auto;
+  cm::optional<Encoding> encoding;
   if ((name == "UTF8") || (name == "UTF-8")) {
     encoding = UTF8;
   } else if (name == "NONE") {
     encoding = None;
   } else if (name == "ANSI") {
     encoding = ANSI;
+  } else if (name == "AUTO") {
+    encoding = Auto;
   } else if (name == "OEM") {
     encoding = OEM;
   }

Source/cmProcessOutput.h

@@ -8,6 +8,8 @@
 #include <string>
 #include <vector>
 
+#include <cm/optional>
+
 /** \class cmProcessOutput
  * \brief Decode text data to internal encoding.
  *
@@ -31,7 +33,7 @@ public:
    * \param name a encoding name.
    * \return encoding enum value or Auto if \a name was not found.
    */
-  static Encoding FindEncoding(std::string const& name);
+  static cm::optional<Encoding> FindEncoding(std::string const& name);
 
   /// The code page that is used as internal encoding to which we will encode.
   static unsigned int defaultCodepage;