If the user explicitly requested gnutls or libgcrypt generate an error if they
are not found.

Accurately describe the default behavior: Wireshark will use the libraries if
they are available.

May help avoid problems like that described in:
https://ask.wireshark.org/questions/35600/not-able-to-configure-wireshark-with-gnutls

Change-Id: I5f0b2ce87cb7c6b337de93609f13d8027e662263
Reviewed-on: https://code.wireshark.org/review/3750
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>