Mouse events were introduced by commit 7fbdfbd4d4
which states in the commit message that MOUSE_EVENTS was also renamed to
BUILD_MOUSE_EVENTS. However, this renamal seems to have been incomplete, as the
respective option in cmake/config.h.in was left to MOUSE_EVENTS. This commit
fixes this.
Closes: https://github.com/brndnmtthws/conky/issues/1380
Signed-off-by: Jaak Ristioja <jaak@ristioja.ee>