Port package to C99.

Reached out to upstream but no reply and no public bug tracker.
https://fedoraproject.org/wiki/Toolchain/PortingToModernC
Tracked at https://bugzilla.redhat.com/show_bug.cgi?id=2158327

diff --git a/ruby/caca-canvas.c b/ruby/caca-canvas.c
index 5dc66abbe1f84f68..6672313f277b981d 100644
--- a/ruby/caca-canvas.c
+++ b/ruby/caca-canvas.c
@@ -16,6 +16,7 @@
 #include <errno.h>
 #include "caca-dither.h"
 #include "caca-font.h"
+#include "caca_internals.h"
 #include "common.h"
 
 VALUE cCanvas;
diff --git a/ruby/caca-display.c b/ruby/caca-display.c
index ac29dafd3995f46f..6e4cfa20b3fbf782 100644
--- a/ruby/caca-display.c
+++ b/ruby/caca-display.c
@@ -226,7 +226,7 @@ static VALUE get_event(VALUE self, VALUE event_mask, VALUE timeout)
     return e;
 }
 
-static VALUE driver_list(void)
+static VALUE driver_list(VALUE unused)
 {
     VALUE ary;
     char const* const* list;
diff --git a/ruby/caca-font.c b/ruby/caca-font.c
index 705f928c6da45fba..72b56298c0f82bf2 100644
--- a/ruby/caca-font.c
+++ b/ruby/caca-font.c
@@ -41,7 +41,7 @@ static VALUE font_initialize(VALUE self, VALUE name)
     return self;
 }
 
-static VALUE font_list(void)
+static VALUE font_list(VALUE unused)
 {
     VALUE ary;
     char const* const* list;