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 jumble. Should have split it up.