Created attachment 1938844 [details]
linsmith-c99.patch

I started patching things, but the list goes on and on:

callbacks.c:103: implicit function declaration: import_csv_activate
callbacks.c:676: implicit function declaration: import_s2p_sel_all_btn_clicked
callbacks.c:684: implicit function declaration: import_s2p_sel_none_btn_clicked
log.c:283: implicit function declaration: ok_cancel_dialog
log.c:280: implicit function declaration: run_filedialog
callbacks.c:668: implicit function declaration: Imports2pWindow_realize
log.c:276: implicit function declaration: ok_dialog
callbacks.c:692: implicit function declaration: import_s2p_sel_invert_btn_clicked
callbacks.c:716: implicit function declaration: import_s2p_impS22_btn_clicked
import.c:543: implicit function declaration: loadlist_clear
callbacks.c:700: implicit function declaration: import_s2p_cancel_btn_clicked
import.c:538: implicit function declaration: number_loads
import.c:496: implicit function declaration: load_append
callbacks.c:638: implicit function declaration: switch_load_mode
callbacks.c:708: implicit function declaration: import_s2p_impS11_btn_clicked
import.c:77: implicit function declaration: s2p_file_loaded
callbacks.c:119: implicit function declaration: loglist_export
callbacks.c:298: implicit function declaration: load_rx_valid
callbacks.c:111: implicit function declaration: s2p_files1_activate
callbacks.c:573: implicit function declaration: loglist_initialize
callbacks.c:306: implicit function declaration: active_load_rx_remove
callbacks.c:288: implicit function declaration: on_load_selected
log.c:146: implicit function declaration: number_loads
callbacks.c:415: implicit function declaration: load_nb_valid
callbacks.c:423: implicit function declaration: active_load_nb_update
callbacks.c:322: implicit function declaration: active_load_rx_update
callbacks.c:389: implicit function declaration: load_nb_modified
load.c:298: implicit function declaration: loadlist_nb_clear
load_nb.c:70: implicit function declaration: number_selected_loads
load.c:296: implicit function declaration: loadlist_rx_clear
load.c:376: implicit function declaration: xml_file_loaded
printer.c:70: implicit function declaration: create_printer_chart
element.c:830: implicit function declaration: xml_file_loaded
printer.c:71: implicit function declaration: recalculate_all
load.c:265: implicit function declaration: on_load_nb_selected
load_rx.c:70: implicit function declaration: number_selected_loads
load.c:222: implicit function declaration: enable_load_nb_buttons
cables.c:234: implicit function declaration: scale_entry_pair_set
load_nb.c:211: implicit function declaration: enable_load_buttons
load.c:262: implicit function declaration: on_load_rx_selected
load_rx.c:56: implicit function declaration: load_rx_valid
load_rx.c:185: implicit function declaration: enable_load_buttons
element.c:310: defaulting to int
load.c:221: implicit function declaration: enable_load_rx_buttons
chart.c:656: implicit function declaration: clear_element_lnlist

So I think we just need to build in C89 mode.