Fix configure function checks for recent clang versions

Simply try taking the address of the function instead of doing funny
stuff with the pointer which clang doesn't like anymore.

This works with `-std=c89 -pendantic` and anything more permissive. On
recent versions of clang works while producing -Wstrict-prototypes due
to function declaration without a prototype being deprecated).

configure

@@ -503,9 +503,8 @@ check_string_function()
 	msg_checking "for function $1"
 	string_function_code="
 #include <string.h>
-int main() {
-	return $1;
-}
+int (*ptr)() = (int (*)()) &$1;
+int main() { return 0; }
 "
 	if try_compile_link "$string_function_code"
 	then

scripts/checks.sh

@@ -495,7 +495,7 @@ check_function()
 	__func="$1"
 	shift
 	msg_checking "for function $__func"
-	if try_compile_link "char $__func(); int main(int argc, char *argv[]) { return ((int*)(&$__func))[argc]; }" "$@"
+	if try_compile_link "extern int $__func(); int (*ptr)() = &$__func; int main() { return 0; }" "$@"
 	then
 		msg_result yes
 		return 0