openssl: use += for LDFLAGS, drop env PC docs

openssl: use += for LDFLAGS, drop env PC docs

Using `LDFLAGS=` instead of `LDFLAGS+=` will stomp over an earlier
setting of LDFLAGS, and the DMARC support is now further up in
`src/EDITME`, thus likely to get stomped upon.

Rather than continue to document using `PKG_CONFIG_PATH` via env, the
in-Local/Makefile support has been around for a little while now, so go
ahead and make that the only way we suggest here.

Add a mention of _why_ we use both `USE_OPENSSL_PC` and `LDFLAGS`.