update Makefile for internal structures

The new Makefile now compiles the currently existing internal structures
needed for the syntax analysis. A new variable (PROGRAM_OBJECTS) was
also introduced to facilitate the addition of new targets.
remove const qualification of symbol_table_get()

The const qualification of this function was not necessary and would
make things harder.
Merge branch 'dev' into symbols
add symbol_table_get_from_index()
first symbol saving implementation

The current implementation supports the declaration and the definition
of integers. Stencils are partially supported. Arrays and function
identifiers are not yet saved.
Merge branch 'symbols' into dev

The symbol table stores most identifiers.