security/heimdal*: Remove LLVM_DEFAULT build prerequisite

Adjust ./configure to set the correct CLANG_FORMAT value when
clang-format is not found (when none of the llvm ports are installed).

PR: 267814
Submitted by: Tatsuki Makino <tatsuki_makino@hotmail.com>

(cherry picked from commit 22a683a337efe7169b61de8c9ec63e2c0d561891)