gearman: configure enables mysql by default

The **configure** option to enable building gearman `with-mysql` is on by default, thus the **formula** option to build with mysql has no effect (apart from installing mysql if you don't already have it).

Further to this when building gearman on a system that does not have the mysql headers installed the build fails:

    ==> ./configure --prefix=/usr/local/Cellar/gearman/1.1.9
    ==> make install    libgearman-server/plugins/queue/mysql/queue.cc:478: error: 'mysql_fetch_row' was not declared in this scope
    libgearman-server/plugins/queue/mysql/queue.cc:484: error: 'mysql_fetch_lengths' was not declared in this scope
    libgearman-server/plugins/queue/mysql/queue.cc:518: error: 'mysql_free_result' was not declared in this scope
    make[1]: *** [libgearman-server/plugins/queue/mysql/libgearman_server_libgearman_server_la-queue.lo] Error 1
    make: *** [install] Error 2

Should the formula option be inverted to be `without-mysql`? See attached patch.

The patch includes disabling the default options for building with `postgresql` and `sqllite3`.

Is a `depends_on :postgres => :recommended if MacOS.version <= :snow_leopard` needed for postgres? PostgreSQL shipped from >= 10.7.

Related issues:

* #13820
* #20175