Error on missing function prototypes with gcc

This makes it an error to not have a prototype in scope for
a function with external linkage.  The flag is only enabled
for gcc due to -Werror=type not working with all compilers.

Originally committed as revision 22313 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2752,6 +2752,7 @@ elif enabled ccc; then
 elif enabled gcc; then
     check_cflags -fno-tree-vectorize
     check_cflags -Werror=implicit
+    check_cflags -Werror=missing-prototypes
 elif enabled clang; then
     check_cflags -Qunused-arguments
 elif enabled armcc; then