Implement Mario's suggestions:
- Clean up code replace merge algorithm with a function.
- Fix for_statement setting control variable to not a constant.
- Fix for_statement checking BY expression to get correct value for optimization
branch.

Thanks Mario.

Fix constant_folding missing call. [Bug found by Mario.]

Fix divide by 0 bug.

merge

Fix CPPFLAGS Makefile.am for all stage4.

Remove redundant class for get variable name.

Fix constant propagation alg. in for statement like Mario suggestion.

Add note about How Constant Folding class is extended with a implementation constant propagation algorithm
by Mario de Sousa.

Add constant propagation algorithm for loop cycles.

Update symbol_c::first_file/last_file entries in list_c::add_element()

moved class to obtain datatype name (in char *) to get_datatype_info_c

merge

Fix comment about meet semilattice rules.

Complete fix initialize const_value.

Start to implement constant propagation algorithm.

Fix const_value field initialized.

Remove Ambiguous enumerate value in enum conversion functions.

Fix enum conversion function to string.

Fix struct generic_function_call_t initialize style.

Merge with Mario's changes.

Merge with Mario's repository.

Fix warning about call fprintf without format string.

merge

Remove Ambiguous enumerate value in enum conversion functions.

Fix enum conversion function to string.

Fix struct generic_function_call_t initialize style.

Small code cleanup.

Allow bison to ask for an END_OF_INPUT token multiple times!

Protect against redefining macros

merge

Fix flag to enable conversion functions.

Code cleanup: move tracking functions to flex.

Insert the auto generated enum datatype conversion functions after the TYPE...END_TYPE, instead of at the end of input file.

merge

Set ENO to false when conversion function fails.

Fix typo in functionDataType array.

Add files missing from a few commits ago...

merge

Refix automake files.

Add some comments to argument new code.

Start to change code like Mario suggestions.

Create functionDataType array in derived_conversion_functions_c.

Add conversion functions option.

Start enumerate conversion functions.

merge

Be sure to not redefine TRUE and FALSE macros.

merge

Removed unused __attribute__((__packed__)) only gcc compatible.

Protect IEC_STD_LIB.H header agaist multi-include problems.

merge

Start to move enumeration declaration checking to its own class.

Fix comments and code alignment

Fix warnings previous change introduced when compiling with gcc.

Fix some important clang++ warnings.

Allow use of type#value syntax when defining default initial value inside a TYPE..END_TYPE declaration.

Do datatype verification of fully qualified enumerated values (e.g. ENUMTYPE#enumconst)

Add datatype checking of enumeration data type declarations.

Print const values of symbol.

Fix code generation of enum value constants (this is a temporary fix - it will be followed by a code cleanup).

Fix bug in datatype checking of enumerated datatypes delcarations.

Fix datatype checking of enumerated datatype declarations. We now follow the fill->narrow algorithm correctly.

Add method to determine the id (name) of a datatype.

Change the base datatype of enumerations to an enumerated_type_declaration_c (for named types!)

Change the public interface to debug functions.

Check for errors in enum datatype declarations. (This is not yet complete as it will not yet stop the compilation process if these errors are found)

Fix one more bug - upper_bound() works a little differently to what we want!

Fix more bugs in dsymtable_c (real semantics of std::multimap::lower_bound is not what was assumed!)

Fix counting bug in dsymbtable_c

Use duplicate symtable instead of symtable for enum constant value table (this will later allow us to detect semantic errors in IEC 61131-3 source code)

Move the enumerated_value_symtable to fill_candidate_datatypes_c, since it is only used there!

Make search_base_type_c a singleton!

Start datatype checking of variable declarations (for now, only seting symbol.datatype of enumerations)

Do datatype checking of enum values defined inside anonymous enumeration types (i.e. enum types defined inside a VAR ... END_VAR daclaration).

Added support for testing BCD value validity in BCD_TO_UINT function

Fixed bug in DATE_AND_TIME_TO_TIME_OF_DAY and DATE_AND_TIME_TO_DATE when date and time is before 1970-01-01-00:00:00

Fix warning message when declaring global variables in configuration

Removed unused variable

Replacing libc time functions for date <-> timespec translation by platform and localtime independant implementation

Removed files generedted by autoconf, should not be in repo

Fixed timegm usage incompatible with mingw, reverted to mktime in case mingw is detected

Fix in accessor undefined init methods for function block global instances

Fix undefined function toupper in iec_std_lib

Adding support for declaring function block instances as global and external variables

Fix DATE conversions standard functions

Fix definition of BCD to ANY_UINT and ANY_UINT to BCD conversion standard functions

Fix indentation in SFC SL qualifier code generated

Fix indentation and unused variable warning in SFC generated code

Fix pragma with double bracket token stripping

Fix some warning about loss of precision data in iec_std_lib.h file.

Fix typo.

Remove deprecated class search_constant_type_c

replace call to search_constant_type_c with symbol.datatype

Removing several warning in iec_std_lib.h

Code formating only.

Fix stage4 for compare expressions.

Prepare to delete search_constant_type_c -> Move the static variables from search_constant_type_c to get_datatype_info_c