Need indication of absence of pkg-config for the cmake 3.4 build script

[metatoaster]

In a Debian 8 environment, the package pkg-config is not fetched along with build-essential, and find_package does not indicate that the functionality is not available, leaving the users who already have the package libssl-dev installed in a rather confused state until they figure this out. Would be useful if this issue is indicated to the user trying to build this thing.