ENH: Restored APPEND option to EXPORT() command in new implementation.

Source/cmExportCommand.h

@@ -65,7 +65,7 @@ public:
     {
     return
       "  export(TARGETS [target1 [target2 [...]]] [NAMESPACE <namespace>]\n"
-      "         FILE <filename>)\n"
+      "         [APPEND] FILE <filename>)\n"
       "Create a file <filename> that may be included by outside projects to "
       "import targets from the current project's build tree.  "
       "This is useful during cross-compiling to build utility executables "
@@ -73,6 +73,8 @@ public:
       "them into another project being compiled for the target platform.  "
       "If the NAMESPACE option is given the <namespace> string will be "
       "prepended to all target names written to the file.  "
+      "If the APPEND option is given the generated code will be appended "
+      "to the file instead of overwriting it.  "
       "If a library target is included in the export but "
       "a target to which it links is not included the behavior is "
       "unspecified."
@@ -88,6 +90,7 @@ public:
 private:
   cmCommandArgumentGroup ArgumentGroup;
   cmCAStringVector Targets;
+  cmCAEnabler Append;
   cmCAString Namespace;
   cmCAString Filename;
 };