depends: Add documentation for FORCE_USE_SYSTEM_CLANG make flag

Author: dongcarl

depends/README.md

@@ -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