Feature: Check for submodules at configure time.

[patha454]

Many users who are not familiar with Git and C are attempting to build this implementation without checking out the Git submodules. They encounter build errors which they are unable to diagnose.

This PR adds a basic sanity check to CMake, to test if the submodules are present at configuration time. If the submodules are not present or do not appear to be in order, CMake will fail with an informative error message.

Examples of issues raised by users unfamiliar who have not checked out submodules:

• https://github.com/KhronosGroup/OpenVX-sample-impl/issues/14
• https://github.com/KhronosGroup/OpenVX-sample-impl/issues/36
• https://github.com/KhronosGroup/openvx-samples/issues/6 (Possibly attempting to use the OpenVX API with missing headers after a minced build of this implementation.)

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #38 (2fde606) into openvx_1.3 (b94f478) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##           openvx_1.3      #38   +/-   ##
===========================================
  Coverage       76.12%   76.12%           
===========================================
  Files             138      138           
  Lines           23470    23470           
===========================================
  Hits            17866    17866           
  Misses           5604     5604           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b94f478...2fde606. Read the comment docs.