Fix bug #973: configuration option to control case-sensitivity during filename comparison.

This commit basically takes the approach to filename comparison used under Win32 (use of "strncasecmp()" instead of "memcmp()") and makes it available on other systems, controlled by a configuration option ("xdebug.case_sensitive_fs"). The default value is 1, which causes the old, case-sensitive behavior. The Win32 code is left untouched.

This addresses [bug #973](http://bugs.xdebug.org/view.php?id=973) ("Breakpoints not triggered on case-insensitive file systems in files autoloaded by spl_autoload") by allowing a developer to change the behavior of xdebug to match the behavior of the filesystem.

The main thing that's not included is a test, which I wasn't sure how to implement.