Fix build on GCC 4.9

Fix build on GCC 4.9

Seems like GCC 4.9 likes to warn about unused functions:

- Remove unused function linintr_solve()
- Remove unused function endian_char()
- Remove unused function implode()
- Remove unused copy of function read_active_macros()
- Remove unused function format_type()
- Make function seemingly poking some stuff in int24.cpp global (never called)
- Remove unused data numbers_l
- Remove unused data numbers_u
- Remove unused function print_decimals()
- Remove unused function format_float_base()
- Remove unused function substr()
- Remove unused function get_levelsize()
- Remove unused function default_headersize()
- Remove unused function get_button_name()
- Remove unused function get_axis_name()
- Remove unused function pick_volume()
- Remove unused function UI_controller_index_by_logical()
- Remove unused function movie_path()
­ Remove unused function clear_clipboard()
- Remove unused function string_pad()