According to the pkg-config manpage, the "Libs" line in a .pc file
should give the link flags "specific to your package", and specifically
says not to include link flags for dependencies:
    Libs:  This line should give the link flags specific to your
           package.  Don't add any flags for required packages;
           pkg-config will add those automatically.

This commit removes the hardcoded dependency in the libunbound
pkg-config .pc file on the libcrypto and libssl modules and instead
populates the .pc file based on which crypto library was selected at
configure time.
Note that the .pc file specifies pkg-config module names for the
"Requires" line and this can vary from the library filename (e.g. "nss"
is the pkg-config module name vs. "nss3" being the library name).