security/heimdal-devel: fix with stock clang and lld >= 16

Upstream clang >= 16 made -Wimplicit-function-declaration into an error
by default. In the base system, this change was reverted to reduce the
fallout in ports, because there are many problematic configure scripts.

For security/heimdal-devel this also applies, so for building the port
with devel/llvm16 or higher we need to add
-Wno-implicit-function-declaration to CFLAGS.

Additionally, similar to bug 275979, add -Wl,--undefined-version to
LDFLAGS, to suppress any linker errors about missing versioned symbols.

PR: 283132
Approved by: cy (maintainer)
MFH: 2024Q4

(cherry picked from commit c1d61be19caf5dd7c4dee1fc0f60c76d8aab3b64)