Since the secp256k1 subtree update in bitcoin/bitcoin#19228, it hasn't been possible to cleanly compile Bitcoin Core on OpenBSD. Building fails with (excerpt from just building libsecp, cc is Clang 8.0.1):

./autogen.sh
./configure CC=cc MAKE=gmake
gmake -j6 V=1
...
gmake
gcc -I. -I./src -Wall -Wextra -Wno-unused-function -g -c src/gen_context.c -o gen_context.o
In file included from src/gen_context.c:16:
src/util.h:179: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'uint128_t'
gmake: *** [Makefile:1689: gen_context.o] Error 1

libsecp does some native compiler detection in configure, and then uses that compiler specifically for the ecmult precomputation. i.e:

The problem is that this ends up picking up and using OpenBSDs GCC 4.2.1, which barfs when it gets to util.h:

I realise there might not be much that can be done here, given that it's the AX_PROG_CC_FOR_BUILD macro that is returning gcc to use. However thought I'd open an issue here for any thoughts, before we update our docs. I've suggested that affected users can just pass CC_FOR_BUILD=cc to configure when building. Issue here: bitcoin/bitcoin#19559.