ENH: Add a method to remove environment variables

Source/cmSystemTools.cxx

@@ -1280,6 +1280,11 @@ bool cmSystemTools::PutEnv(const char* value)
   return ret == 0;
 }
 
+bool cmSystemTools::UnsetEnv(const char* value)
+{
+  return unsetenv(value) == 0;
+}
+
 std::vector<std::string> cmSystemTools::GetEnvironmentVariables()
 {
   std::vector<std::string> env;

Source/cmSystemTools.h

@@ -291,9 +291,14 @@ public:
   */
   static std::string RelativePath(const char* local, const char* remote);
 
-  /** put a string into the environment
+  /** Put a string into the environment
       of the form var=value */
   static bool PutEnv(const char* value);
+
+  /** Remove an environment variable */
+  static bool UnsetEnv(const char* value);
+
+  /** Get the list of all environment variables */
   static std::vector<std::string> GetEnvironmentVariables();
 
   /** Setup the environment to enable VS 8 IDE output.  */