Fix wrong Header-only usage message

[LilyWangLL]

This regression is from PR #1368.
The wrong message as below:

bext-sml is header-only and can be used from CMake via:

  find_path(BEXT_SML_INCLUDE_DIRS ")boost/sml.hpp")
  target_include_directories(main PRIVATE ${BEXT_SML_INCLUDE_DIRS})

[BillyONeal]

In general, I think when we are fixing regressions like this I think we should be adding tests for the regression unless we can show that it isn't reasonable to do so. Clearly, we've demonstrated that's a bug we might write :)

In this case I had discovered the same bug and fix over in #1405 so I already had a test handy so I just moved it over here.

[BillyONeal]

Thanks!