Do not add -Werror when --enable-debug is used

Using -Werror causes some problems due to warnings in the SDL_sound
code, so let's disable it.

We patch configure directly, because:

 1/ configure.in doesn't autoreconf properly out of the box.

 2/ the length of the configure or configure.in patch would be exactly
    the same (one line), or maybe more for the configure.in patch if
    we want the package to autoreconf properly

 3/ the patch will never go upstream, because upstream is dead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure
===================================================================
--- a/configure
+++ b/configure
@@ -19561,7 +19561,6 @@
   else
     CFLAGS="-O0"
   fi
-  CFLAGS="$CFLAGS -Werror"
 
 cat >>confdefs.h <<\_ACEOF
 #define DEBUG 1