Add support for functions returning VOID (i.e. non-standard extension, allowing functions that do not return any data)

Add -warn flag when running flex -> warn of potential bugs

Add support for non-standard VOID datatype. May ony be used to define functions returning VOID.

Delete the YY_NO_UNPUT define, as we do use the unput() function.

Add support for non standard feature: allow POUs with no in, out and inout parameters

Add option to not add the EN and ENO parameters to every function and FB (generated C code becomes smaller, and uses less memory for function invocations)

Correctly identify errors when parsing erroneous code (make sure flex goes back to INITIAL state when code contains errors that do not allow determining whether ST or IL is being parsed)

Created new branch fix_compilation_warnings

fix compilation error about missing braces

IEC_TIMESPEC has two fields tv_sec and tv_nsec.

Compilation error:
./build/POUS.c:600:45: error: missing braces around initializer [-Werror=missing-braces]
static const STEP temp_step = {{0, 0}, 0, {0, 0}};
^
compilation terminated due to -Wfatal-errors.

fix warning about overflow if matiec is running on 64-bit platform,
but the generated C code will cross-compiled for 32-bit platform

For example:
./build/config.c:29:39: error: large integer implicitly truncated to unsigned type [-Werror=overflow]
unsigned long greatest_tick_count__ = 18446744073709551611UL; /*tick*/
^
compilation terminated due to -Wfatal-errors.
cc1: all warnings being treated as errors