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

While here, use LDFLAGS+= for -Wl,--undefined-version, to avoid
overwriting any user-specified LDFLAGS.

PR: 283131
Approved by: maintainer timeout (2 weeks)
MFH: 2025Q1