No CMake install target when Crossguid is not the root project

[merwaaan]

Hi @graeme-hill

When using CMake, no INSTALL target is created if crossguid is not the root project.
We're in this situation because we use crossguid as a submodule of a parent CMake project (however we still need the INSTALL target).

This seems to be due to this line:

crossguid/CMakeLists.txt

Line 58 in ca1bf4b

if (${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})

Why is that condition required?
I've seen other projects adding an option to toggle installation on or off (eg. "CROSSGUID_INSTALL_ENABLED").
I could propose a PR with something similar if that's ok for you.