This was evidently missed in commit 6337865f3, which generally did
s/TRUE/true/ everywhere. It escaped notice up to now because ICU
versions before ICU 68 provided definitions of "TRUE" and "FALSE"
regardless. With ICU 68, it fails to compile.
Per report from Condor. Back-patch to v11 where 6337865f3 came in.
(I've not tested v10, where this call originated, but I imagine
it's fine since we defined TRUE in c.h back then.)
Discussion: https://postgr.es/m/7a6f3336165bfe3ca66abcda7966f9d0@stz-bg.com