Add configure tests for stdbool.h and sizeof bool
This will allow us to assess how many platforms have bool with a size
other than 1, which will help us decide how to go forward with using
stdbool.h.
Discussion: https://www.postgresql.org/message-id/flat/
3a0fe7e1-5ed1-414b-9230-
53bbc0ed1f49@2ndquadrant.com