Attempted fix for install of header-only libraries

[isherman]

When trying to invoke a make install with a medium-sized parent project (path is redacted here as <...>) I encountered the following error, related to the header-only "core/struct" library:

CMake Error at <...>/farm_ng_core/cpp/farm_ng/core/struct/cmake_install.cmake:46 (file):
  file INSTALL cannot find
  "<...>/farm_ng_core/cpp/farm_ng/core/struct/base":
  No such file or directory.
Call Stack (most recent call first):
  <...>/farm_ng_core/cpp/farm_ng/core/cmake_install.cmake:57 (include)
  <...>/farm_ng_core/cpp/cmake_install.cmake:52 (include)
  <...>/farm_ng_core/cmake_install.cmake:57 (include)
  cmake_install.cmake:47 (include)

This attempted fix puts more of the install logic behind the condition that we're building a library with headers + sources.

It works for my specific use case; admittedly I have not made the effort to understand all of the surrounding logic in this macro.

[strasdat]

Looks good, all CI passes so there is little risk