Fix installing CMake exported targets

[stanionascu]

Before the patch, the installed exported targets were referencing artifacts produced in the build tree, the patch fixes:

• the exported target to also provide the include dir
• make the installed package relocatable

E.g.:

find_package(yaml-cpp REQUIRED)

add_executable(${PROJECT_NAME} main.cpp)
target_link_libraries(${PROJECT_NAME} yaml-cpp)

Previously it would fail if the build tree would be removed.

[stanionascu]

• Replaced target_include_directories with set_property(TARGET yaml-cpp) to support older cmake versions.

[stanionascu]

Superseeds #446 as it tries to support cmake <2.8

[sergiud]

How does this PR supersede #446 if it also uses generator expressions ($<BUILD_INTERFACE:...> and $<INSTALL_INTERFACE:...>) not available until CMake 2.8.11? The same release introduces target_include_directories which you replaced by set_property. Not sure how your PR is supposed to support CMake prior to version 2.8.11.

[stanionascu]

@sergiud you were right with the generator expressions. now the main change is in yaml-cpp-config.cmake.in , which in case of using a newer cmake version will propagate the include dir onto the imported target (but the yaml-cpp can still be built and deployed using cmake 2.6)

[mwoehlke-kitware]

I noticed this outsatnding PR after submitting #538, however, it seems to have some issues compared to #538. (Also, #538 additionally fixes a bug with the Windows library install location.)

[mwoehlke-kitware]

I'm not sure this is correct; include_directories is usually passed an absolute path. At best, this change is superfluous.

[mwoehlke-kitware]

This should change also.

[mwoehlke-kitware]

This will expand to <prefix>/<prefix>/... in some cases.

[mwoehlke-kitware]

This should be set on the target, not done like this.

[SGSSGene]

I am assuming this PR is long outdated (targeting cmake below 3.15 is definitely not on our to do list any more 😅 )

Please feel free to reopen if any future work is planed.