autoconf: Add some warning flags for clang 13 or newer

-Wnull-pointer-subtraction
-Wunused-but-set-parameter
-Wunused-but-set-variable

Author: fxlb

aclocal.m4

@@ -670,11 +670,14 @@ AC_DEFUN(AC_LBL_DEVEL,
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wmissing-noreturn)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wmissing-prototypes)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wmissing-variable-declarations)
+		    AC_LBL_CHECK_COMPILER_OPT($1, -Wnull-pointer-subtraction)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wpointer-arith)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wpointer-sign)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wshadow)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wsign-compare)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wstrict-prototypes)
+		    AC_LBL_CHECK_COMPILER_OPT($1, -Wunused-but-set-parameter)
+		    AC_LBL_CHECK_COMPILER_OPT($1, -Wunused-but-set-variable)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wunused-parameter)
 		    AC_LBL_CHECK_COMPILER_OPT($1, -Wused-but-marked-unused)
 		    # Warns about safeguards added in case the enums are