Detect the directory of extension's source after extracting the ext archive

Some extensions' source files are not immediately in the <extname> directory of the pecl archive. For example the xhprof ext source files are in xhprof-<version>/extension/ . The pear installer resolves this by detecting the directory of config.m4 so I used the same approach in ExtensionInstaller. This pull request should fix   Issue #164

Also xhprof does not appear to be a zend_extension anymore (adding it with zend_extension=xhprof.so raises a warning)