Added a little note about how to set environmental variables (for people like myself).

Author: awdavies

doc/howto/build.txt

@@ -1,5 +1,6 @@
 By default, all libraries mentioned below (except Windows/DirectX SDK) are assumed to be in the directory
-defined by XRAY_16X_LIBS environment variable.
+defined by XRAY_16X_LIBS environment variable (set this by going to System, then going to Advanced system
+settings and then clicking on Environmental Variables).
 
 To build X-Ray Engine you'll need following libraries:
   * OpenAutomate: https://developer.nvidia.com/openautomate