c2nim not work very well :-(
$ c2nim --dynlib:libgd --cdecl ./gd.h --out:gdtest.nim
/home/hdias/development/tmp/gd.h(14, 62) Warning: comment
'version605b5d1778' ignored [CommentXIgnored]
/home/hdias/development/tmp/gd.h(15, 62) Warning: comment
'version605b5d1778' ignored [CommentXIgnored]
/home/hdias/development/tmp/gd.h(16, 62) Warning: comment
'version605b5d1778' ignored [CommentXIgnored]
/home/hdias/development/tmp/gd.h(17, 62) Warning: comment
'version605b5d1778' ignored [CommentXIgnored]
/home/hdias/development/tmp/gd.h(22, 58) Error: expected ';'