libflac installs its own assert.h which fails to define assert()

Define assert() to no-op
(cherry picked from commit 1e2a142ff8e90af6a4ab3fa725320a1ab2772eab)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/flac.c

@@ -26,6 +26,9 @@
  * Preamble
  *****************************************************************************/
 
+/* workaround libflac overriding assert.h system header */
+#define assert(x) do {} while(0)
+
 #ifdef HAVE_CONFIG_H
 # include "config.h"
 #endif