Update separate-compilation.md

extern variable mention removed

Author: VictorEijkhout

sources/modules/compilation-model/separate-compilation.md

@@ -82,7 +82,6 @@ A student should be able to:
 2. Use compile flags, including `-I` and `-L` to specify search paths.
 3. Make header files for their own code, including using header guards.
 4. Use include directives for the headers of external libraries.
-5. Be able to declare `extern` variables.
 
 #### Caveats