Improve makefile and fix build

This PR comes in three parts:

* Improve Makefile:
    * make clean now removes all generated files
    * use awesome make syntax to make the makefile more concise

* Remove versioned generated files:
   * remove the flex and bison generated files from git

* Fix a problem with building the language
    * change <malloc.h> to <stdlib.h> in `libcs/cs_utf8.c`

Sorry for the jumbled PR. Should have split it up.