Issue 595

[rollingslack]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any): partially addresses libiperf catchup #595

• Brief description of code changes (suitable for use as a commit message): Add libiperf api for getting iperf version

Notes

• The extended version info (CJSON, system info, et al) is not present with this API.
• Makefile.am has changes to account for a new api test file. I've not added the changes to Makefile.in - per Provide a "--disable-profiling" option with configure  #760 (comment). So, this will require running the bootstrap.sh and regenerating the Makefile.in and merging before it can go into master.
  If I need to
• I've introduced a new placeholder for API tests as t_api.c - this also runs on make check. This is not really required for this simple API change, but is probably useful for housing/adding on other API check tests later.

[bmah888]

Looks good, thanks.