cmake: only enable doxygen when configured to
cmake: only enable doxygen when configured to

Also use find_package(... REQUIRED) pattern to be a bit more concise.

Signed-off-by: GitHub <1505422b2465e9a84f6fdfaa161078890c593f06@github.com>

alanxz/rabbitmq-c,alanxz/rabbitmq-c,alanxz/rabbitmq-c