MySQL support should not be magically detected (xbmc-pvr-addons fails to build)

MySQL installed on the build host would get autodetected even if `MYSQL_SUPPORT` is not set to yes. And then xbmc-pvr-addons fails to build with:

    libcmyth/cmyth_local.h:39:25: fatal error: mysql/mysql.h: No such file
or directory

To reproduce, I guess you can have MySQL installed on the host, and `MSYQL_SUPPORT=no` in your config. MySQL will not get pulled in as a dependency when building OpenELEC, but building xbmc-pvr-addons would fail. That's on Gentoo 64-bit.