message: depends: Add documentation for FORCE_USE_SYSTEM_CLANG make flag
diff: @@ -109,6 +109,10 @@ The following can be set when running make: `make FOO=bar`
  <dd>Optional salt to use when generating host package ids</dd>
  <dt>BUILD_ID_SALT</dt>
  <dd>Optional salt to use when generating build package ids</dd>
  +<dt>FORCE_USE_SYSTEM_CLANG</dt>
  +<dd>(EXPERTS ONLY) When cross-compiling for macOS, use clang found in the
  +system's <code>$PATH</code> rather than the default prebuilt release of clang
  +from llvm.org</dd>
  </dl>
  If some packages are not built, for example `make NO_WALLET=1`, the appropriate
  