I'm using Alpine 3.14 (under WSLv2) to try to build Chapel in the preferred configuration. It looks like check_llvm_config (and the entire build process) assumes a setup that Alpine's clang/llvm11 doesn't match.
Specifically, clang and llvm11 are installed in unrelated directories, so the directory llvm-config --includedir reports (/usr/lib/llvm11/include) doesn't contain clang/Basic/Version.h (it doesn't have a clang subdirectory at all). /usr/include/clang/Basic/Version.h exists, and looks like the right thing, so explicitly making /usr/lib/llvm11/include/clang symlink to /usr/include/clang makes the prechecks pass and the build find the necessary #define-d constants.
Once I've done that, I also need to use apk to add clang-static, not just clang-dev. And set CC=clang and CXX=clang++ (if I don't do that, it tries to find clang & clang++ in an llvm rooted directory where they're not installed at a later stage of the build), then edit third-party/qthread/qthread-src/src/hashmap.c and third-party/qthread/qthread-src/src/qloop.c to manually add #include <sys/types.h> (because they both use ssize_t without including any of the headers that provide it).
This was decidedly unintuitive, and I'm trying to figure out if I missed anything that would have simplified it.