Remove obsolete testbuild.c.

It was originally used for probing the available warning flags in the
compiler, by adding them one by one and making sure this test file
still compiled.

But that whole mechanism was removed in commit 306fab356e357ef, and
since then, testbuild.c has been unused. Belatedly throw it away.

cmake/testbuild.c

@@ -1,23 +0,0 @@
-#include <stdio.h>
-#include <assert.h>
-#include <stdlib.h>
-#include <time.h>
-#include <stdarg.h>
-#include <string.h>
-#include <errno.h>
-#include <tgmath.h>
-
-#include <sys/time.h>
-#include <sys/resource.h>
-
-#include <gtk/gtk.h>
-#include <gdk/gdkkeysyms.h>
-
-#include <gdk-pixbuf/gdk-pixbuf.h>
-
-#include <gdk/gdkx.h>
-#include <X11/Xlib.h>
-#include <X11/Xutil.h>
-#include <X11/Xatom.h>
-
-int dummy;