ENH: Enable basic OpenVMS platform support

This adds the Modules/Platform/OpenVMS.cmake platform file for OpenVMS.
We just use Unix-like rules to work with the GNV compiler front-end.

A problem with process execution currently prevents CMake link scripts
from working, so we avoid using them.

Modules/Platform/OpenVMS.cmake

@@ -0,0 +1,7 @@
+INCLUDE(Platform/UnixPaths)
+
+SET(CMAKE_C_CREATE_STATIC_LIBRARY
+  "<CMAKE_AR> cr <TARGET> <LINK_FLAGS> <OBJECTS>"
+  "<CMAKE_RANLIB> <TARGET>"
+  )
+SET(CMAKE_CXX_CREATE_STATIC_LIBRARY ${CMAKE_C_CREATE_STATIC_LIBRARY})

Source/cmGlobalUnixMakefileGenerator3.cxx

@@ -31,7 +31,7 @@ cmGlobalUnixMakefileGenerator3::cmGlobalUnixMakefileGenerator3()
   this->ToolSupportsColor = true;
   this->ForceVerboseMakefiles = false;
 
-#ifdef _WIN32
+#if defined(_WIN32) || defined(__VMS)
   this->UseLinkScript = false;
 #else
   this->UseLinkScript = true;