ABI changes (Bug#60061)

Bumping the version from 16 to 18, instead of 17, was done to avoid a
library conflict on Mac OS X between MySQL 5.1 and MySQL 5.5. In MySQL
5.1 GNU libtool was used, that made the ABI version used in the file
name to be 16, and the one stored inside the binary to be 17.

MySQL 5.5 uses CMake as a build tool, that will store the same ABI
number in the file name as inside the binary in Mac OS X, and then
bumping the ABI number two steps avoids a conflict on Mac OS X.