use mysql_config to retrieve MySQl build options

[remicollet]

Major rewrite of autoconf for MySQL, based on ext/mysqli/config.m4

This make things much more simpler, but this change semantic of option

Before:

[  --with-mysql=DIR        Location of MySQL base directory], yes, no)

After:

[  --with-mysql=FILE       FILE is path to mysql_config command], yes, no)