Enable -Wno-unused-parameter

Instead of adding __attribute__((unused)) to unused arguments, add the
-Wno-unused-parameter compiler flag.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

configure.ac

@@ -30,6 +30,7 @@ AC_ARG_ENABLE([werror], [AS_HELP_STRING([--disable-werror],
 if test "$ac_cv_prog_gcc" = yes; then
    WARN_CFLAGS="-Wall -W -Wshadow -Wstrict-prototypes -Wpointer-arith -Wcast-align -Wcast-qual -Wno-strict-aliasing -Wvla"
    WARN_CFLAGS="$WARN_CFLAGS -Wno-unknown-warning-option -Wno-stringop-truncation"
+   WARN_CFLAGS="$WARN_CFLAGS -Wno-unused-parameter"
    if test "x$enable_werror" != "xno"; then
        WARN_CFLAGS="$WARN_CFLAGS -Werror"
    fi