cmake: make yaml-cpp package relocatable

[sergiud]

This PR makes yaml-cpp-config.cmake relocatable allowing the package to be used both from the build tree and the install path. The changes are backwards compatible.

Typical usage of yaml-cpp-config.cmake:

find_package (yaml-cpp 0.5.3 REQUIRED)

add_executable (myproject main.cpp)
target_link_libraries (myproject yaml-cpp)

The yaml-cpp-config.cmake is now always installed under ${CMAKE_INSTALL_PREFIX}/(lib|lib64)/cmake/yaml-cpp. This change was necessary to simplify the config logic. Previously, yaml-cpp-config.cmake was installed under ${CMAKE_INSTALL_PREFIX}/CMake for Win32 (excluding cygwin), and ${CMAKE_INSTALL_PREFIX}/(lib|lib64)/cmake/yaml-cpp on other platforms. The different config paths that depend on the platform in use are unnecessary.

[ligfx]

+1 This solved my issue with propagating include paths on the yaml-cpp target when built inside a larger target.

I can't speak to the change in yaml-cpp-config.cmake location.

[SGSSGene]

seems to be fixed in $475 or #538.

Please feel free to reopen if issue continues to exists.