Because we compile with "-Werror" each warning is an error. In this
parser an implicit fall-through is used in a switch() statement, which
is typically a bad thing, but not unheard of.

In this code it was checked + verified that it was the intent to fall
through. So the warning was disabled.

The configuration now builds a shared library and links FLEX libraries with
it.

Cranking up the compiler warnings to close to insane:
    - in  "static struct llist *lit()" fix a classic!
      single "&" where "&&" meant:
           inch=input();
        -  while(inch !=EOF & inch != ';' && inch != '\0') {
        +  while(inch !=EOF && inch != ';' && inch != '\0') {
    - removed unused variables
    - renamed shadowed variable/function parameter
    - fix function prototype/function not returning a value
      prototype:
        "char* <function>"
      implementation:
        no return statement at all!

The source code relied on including y.tab.h (the generated header file).
When adding an installable target - such that other projects could (re)use
the vex parser, installing a non-descript file like "y.tab.h" is
undesirable.

To that effect: the generated parser file(s) will now be:
    vex_parser.c vex_parser.h
the generated lexer file will now be:
    vex.yy.c

Use -DCMAKE_INSTALL_DIR=<path> to set desired installation location.
Will install:
    ${CMAKE_INSTALL_DIR}/lib/libvex.a
            ...        /include/vex.h
                                vex++.h
                                vex_parser.h

All code now includes "vex_parser.h" in stead of "y.tab.h"