some warnings with extra compile flags

with extra compile flags, i get these warnings (Windows, msys2 + mingw-w64)
[3/10] Compiling C object md2html/md2html.exe.p/md2html.c.obj
../md2html/md2html.c: In function 'cmdline_callback':
../md2html/md2html.c:299:52: warning: unused parameter 'data' [-Wunused-parameter]
  299 | cmdline_callback(int opt, char const* value, void* data)
      |                                              ~~~~~~^~~~
[4/10] Compiling C object src/libmd4c-html-0.dll.p/md4c-html.c.obj
../src/md4c-html.c: In function 'render_utf8_codepoint':
../src/md4c-html.c:172:54: warning: overflow in conversion from 'int' to 'char' changes value from '239' to '-17' [-Woverflow]
  172 |     static const MD_CHAR utf8_replacement_char[] = { 0xef, 0xbf, 0xbd };
      |                                                      ^~~~
../src/md4c-html.c:172:60: warning: overflow in conversion from 'int' to 'char' changes value from '191' to '-65' [-Woverflow]
  172 |     static const MD_CHAR utf8_replacement_char[] = { 0xef, 0xbf, 0xbd };
      |                                                            ^~~~
../src/md4c-html.c:172:66: warning: overflow in conversion from 'int' to 'char' changes value from '189' to '-67' [-Woverflow]
  172 |     static const MD_CHAR utf8_replacement_char[] = { 0xef, 0xbf, 0xbd };
      |                                                                  ^~~~
../src/md4c-html.c: In function 'md_html':
../src/md4c-html.c:564:41: warning: overflow in conversion from 'int' to 'char' changes value from '239' to '-17' [-Woverflow]
  564 |         static const MD_CHAR bom[3] = { 0xef, 0xbb, 0xbf };
      |                                         ^~~~
../src/md4c-html.c:564:47: warning: overflow in conversion from 'int' to 'char' changes value from '187' to '-69' [-Woverflow]
  564 |         static const MD_CHAR bom[3] = { 0xef, 0xbb, 0xbf };
      |                                               ^~~~
../src/md4c-html.c:564:53: warning: overflow in conversion from 'int' to 'char' changes value from '191' to '-65' [-Woverflow]
  564 |         static const MD_CHAR bom[3] = { 0xef, 0xbb, 0xbf };
      |                                                     ^~~~
[5/10] Compiling C object src/libmd4c-0.dll.p/md4c.c.obj
../src/md4c.c: In function 'md_mark_chain':
../src/md4c.c:109:37: warning: a label can only be part of a statement and a declaration is not a statement [-Wpedantic]
  109 |     #define MD_FALLTHROUGH()        __attribute__((fallthrough))
      |                                     ^~~~~~~~~~~~~
../src/md4c.c:2516:25: note: in expansion of macro 'MD_FALLTHROUGH'
 2516 |         case _T('!'):   MD_FALLTHROUGH();
      |                         ^~~~~~~~~~~~~~
mity / md4c

some warnings with extra compile flags #187
