configure.ac: fix POSIX conformance issues (bug #4261.) Patch from Matt Whitlock: There are actually two distinct classes of problems at play here. On the one hand, libsdl2's configure.ac has some POSIX conformance issues - namely, the use of 'echo -n' and the passage of arguments containing embedded backslashes to 'echo', neither of which is defined by POSIX. The attached patch takes care of these issues.