Stop leaking external headers and symbols to the public API

There are several places where we leak the external headers and libraries to the public library API. We should stop doing that as it forces any user of the library to add correct CFLAGS and LDFLAGS to the compiler and linker.

This is placeholder bug and I will add individual merge requests for every occurrence:

<isc/safe.h> leaks OpenSSL header and symbols (!3215 (merged))
<isc/md.h> leaks OpenSSL header and types (!3218 (merged))
<isc/hmac.h> leaks OpenSSL header and types (!3216 (merged))
...

Edited Mar 12, 2020 by Ondřej Surý

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