Add support for header-only libraries with tribits_add_library()

[bartlettroscoe]

Description

With the move to modern CMake, it should now be easy to support header-only libraries with tribits_add_library(). The idea would be to provide a new keyword HEADERONLY that would allow the SOURCES <src0> <src1> ... argument to be missing. Then an interface-only CMake target would get created.

The hardest parts of adding this new feature is adding a test case for all of the supported use cases and adding documentation.

Motivating user is the STK package in Trilinos.

[bartlettroscoe]

I need to decide how to test this. I order to ensure that all use cases are supported and to make it easy for people to find this example, it might be good to just add tribits_add_library( ... HEADERONLY ...) to one of the existing example/tests projects like TribitsExampleProject or TribitsExampleProject2. The pros/cons of adding this to one of these existing example projects:

• Pro: Ensures gets tested in all use cases (including the generation of exported <Package>ConfigTargets.cmake files).
• Con: May require updates of a lot of existing tests?

I think that if I am crafty, I may not need to update the existing tests much if at all. I will see how this goes.

[bartlettroscoe]

FYI: This addresses #51