For some reason it was previously defined as 0, which is silly.  The only
effect was to disable use of <stdbool.h>, which commit b2328bf6 intended
to make possible.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm%3D3%3DTDYEXUEcHpEx%2BTwc31wo7PA0oBAiNt6sWmq93MW02A%40mail.gmail.com