Build process should check build requirements

[marcjansen]

ol3 has a lot of build requirements. When these are not present, the build fails in strange ways.

The build process should give meaningful messages when these requirements are not met. This includes, for each requirement:

• checking that the requirement is present
• checking that the requirement is a supported version

Refs #318.

[marcjansen]

Commit 858574b adds very basic checking of dependencies without version checking. This might help getting a first idea why the build fails by reporting early that some requirements aren't there.

[marcjansen]

Now that #358 is merged, is this still relevant? Has anybody a better approach?

[elemoine]

I'm failing to understand the link to #358 (Mocha). Also, I think we just need to verify that the deps required for the check target are installed. This excludes jsdoc3.

[marcjansen]

@elemoine. yes #358 is totally unrelated, sorry for the noise.

[twpayne]

Fixed in #448.