cmLinkLineComputer: Extract from cmLocalGenerator

CMake has several classes which have too many responsibilities.
cmLocalGenerator is one of them.  Start to extract the link line
computation.  Create generator-specific implementations of the interface
to account for generator-specific behavior.

Unfortunately MSVC60 has different behavior to everything else and CMake
still generates makefiles for it.  Isolate it with MSVC60-specific
names.

Source/cmMSVC60LinkLineComputer.h

@@ -0,0 +1,18 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#ifndef cmMSVC60LinkLineComputer_h
+#define cmMSVC60LinkLineComputer_h
+
+#include "cmLinkLineComputer.h"
+
+class cmMSVC60LinkLineComputer : public cmLinkLineComputer
+{
+public:
+  cmMSVC60LinkLineComputer(cmState::Directory stateDir);
+
+  std::string ConvertToLinkReference(std::string const& input) const
+    CM_OVERRIDE;
+};
+
+#endif