(CFACT-17) Implement a HTTP client for EC2/GCE facts.

Implementing a simple wrapper around libcurl for performing HTTP
requests.

The HTTP client should be enough for cfacter, but more work will need to
be done to support requirements for a native agent.

EC2 and GCE facts should output an info message iff:
  - They would normally be in the output (i.e. they meet the confines)
  - USE_CURL is not defined

Including a file in the http directory without USE_CURL defined will
result in build errors, so guard the includes with an #ifdef.
I figure when we need the functionality.  There's some TLS stuff we'll need to add to make this a proper client for agent use, but currently this implementation suffices for implementing the remaining facts in cfacter.