Better MySQL dependency detection in percona-toolkit

The percona-toolkit formula was stricly depending on mysql making it impossible to install when a user had maridb or percona-server installed instead of the standard mysql.

It now detects whatever mysql executable exists on the system before requiring the mysql dependency.
Yes it does:

````` shell
$ which mysql_config
/usr/local/bin/mysql_config
$ echo $PATH        
/Users/andreausuelli/.rvm/gems/ruby-2.1.1/bin:/Users/andreausuelli/.rvm/gems/ruby-2.1.1@global/bin:/Users/andreausuelli/.rvm/rubies/ruby-2.1.1/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/andreausuelli/.composer/vendor/bin:/Users/andreausuelli/.rvm/bin
$ mysql_config
Usage: /usr/local/bin/mysql_config [OPTIONS]
Options:
        --cflags         [-I/usr/local/Cellar/percona-server/5.6.19-67.0/include/mysql  -Os -g -fno-strict-aliasing]
        --cxxflags       [-I/usr/local/Cellar/percona-server/5.6.19-67.0/include/mysql  -Os -g -fno-strict-aliasing]
        --include        [-I/usr/local/Cellar/percona-server/5.6.19-67.0/include/mysql]
        --libs           [-L/usr/local/Cellar/percona-server/5.6.19-67.0/lib  -lperconaserverclient  -lssl -lcrypto]
        --libs_r         [-L/usr/local/Cellar/percona-server/5.6.19-67.0/lib  -lperconaserverclient_r  -lssl -lcrypto]
        --plugindir      [/usr/local/Cellar/percona-server/5.6.19-67.0/lib/mysql/plugin]
        --socket         [/tmp/mysql.sock]
        --port           [0]
        --version        [5.6.19-67.0]
        --libmysqld-libs [-L/usr/local/Cellar/percona-server/5.6.19-67.0/lib  -lmysqld   -lssl -lcrypto]
        --variable=VAR   VAR is one of:
                pkgincludedir [/usr/local/Cellar/percona-server/5.6.19-67.0/include/mysql]
                pkglibdir     [/usr/local/Cellar/percona-server/5.6.19-67.0/lib]
                plugindir     [/usr/local/Cellar/percona-server/5.6.19-67.0/lib/mysql/plugin]
`````

Thank you for your quick answers and for following this up.