$ make
make  all-am
make[1]: Entering directory '/home/msamec/Downloads/i3-4.15'
  CC       libi3/libi3_a-dpi.o
  CC       libi3/libi3_a-draw_util.o
  CC       libi3/libi3_a-fake_configure_notify.o
  CC       libi3/libi3_a-font.o
  CC       libi3/libi3_a-format_placeholders.o
  CC       libi3/libi3_a-get_colorpixel.o
  CC       libi3/libi3_a-get_config_path.o
  CC       libi3/libi3_a-get_exe_path.o
  CC       libi3/libi3_a-get_mod_mask.o
  CC       libi3/libi3_a-get_process_filename.o
  CC       libi3/libi3_a-get_visualtype.o
  CC       libi3/libi3_a-ipc_connect.o
  CC       libi3/libi3_a-ipc_recv_message.o
  CC       libi3/libi3_a-ipc_send_message.o
  CC       libi3/libi3_a-is_debug_build.o
  CC       libi3/libi3_a-mkdirp.o
  CC       libi3/libi3_a-resolve_tilde.o
  CC       libi3/libi3_a-root_atom_contents.o
  CC       libi3/libi3_a-safewrappers.o
  CC       libi3/libi3_a-string.o
  CC       libi3/libi3_a-strndup.o
  CC       libi3/libi3_a-ucs2_conversion.o
In file included from libi3/ucs2_conversion.c:12:0:
/usr/include/iconv.h:79:8: error: unknown type name ‘libiconv_open’
 extern iconv_open LIBICONV_DLL_EXPORTED iconv_t (const char* tocode, const char* fromcode);
        ^
In file included from libi3/ucs2_conversion.c:12:0:
/usr/include/iconv.h:79:41: error: ‘libiconv_t’ redeclared as different kind of symbol
 extern iconv_open LIBICONV_DLL_EXPORTED iconv_t (const char* tocode, const char* fromcode);
                                         ^
/usr/include/iconv.h:53:15: note: previous declaration of ‘libiconv_t’ was here
 typedef void* iconv_t;
               ^
/usr/include/iconv.h:89:44: error: expected declaration specifiers or ‘...’ before ‘libiconv_t’
 extern LIBICONV_DLL_EXPORTED size_t iconv (iconv_t cd,  char* * inbuf, size_t *inbytesleft, char* * outbuf, size_t *outbytesleft);
                                            ^
/usr/include/iconv.h:95:47: error: expected declaration specifiers or ‘...’ before ‘libiconv_t’
 extern LIBICONV_DLL_EXPORTED int iconv_close (iconv_t cd);
                                               ^
/usr/include/iconv.h:133:44: error: expected declaration specifiers or ‘...’ before ‘libiconv_t’
 extern LIBICONV_DLL_EXPORTED int iconvctl (iconv_t cd, int request, void* argument);
                                            ^
libi3/ucs2_conversion.c:16:16: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘utf8_conversion_descriptor’
 static iconv_t utf8_conversion_descriptor = (iconv_t)-1;
                ^~~~~~~~~~~~~~~~~~~~~~~~~~
libi3/ucs2_conversion.c:17:16: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘ucs2_conversion_descriptor’
 static iconv_t ucs2_conversion_descriptor = (iconv_t)-1;
                ^~~~~~~~~~~~~~~~~~~~~~~~~~
libi3/ucs2_conversion.c: In function ‘convert_ucs2_to_utf8’:
libi3/ucs2_conversion.c:33:9: error: ‘utf8_conversion_descriptor’ undeclared (first use in this function)
     if (utf8_conversion_descriptor == (iconv_t)-1) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
libi3/ucs2_conversion.c:33:9: note: each undeclared identifier is reported only once for each function it appears in
libi3/ucs2_conversion.c: In function ‘convert_utf8_to_ucs2’:
libi3/ucs2_conversion.c:75:9: error: ‘ucs2_conversion_descriptor’ undeclared (first use in this function)
     if (ucs2_conversion_descriptor == (iconv_t)-1) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
make[1]: *** [Makefile:2081: libi3/libi3_a-ucs2_conversion.o] Error 1
make[1]: Leaving directory '/home/msamec/Downloads/i3-4.15'
make: *** [Makefile:1266: all] Error 2