configure: check and use -Wempty-body

Would have shown the trailing semi-colon in lavu/log.c that broke detection of
256 colour capable terminals.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -4776,6 +4776,7 @@ check_cflags -Wundef
 check_cflags -Wmissing-prototypes
 check_cflags -Wno-pointer-to-int-cast
 check_cflags -Wstrict-prototypes
+check_cflags -Wempty-body
 enabled extra_warnings && check_cflags -Winline
 
 check_disable_warning(){