Inspect handling of private dependencies in install_basic_package_files()

[PeterBowman]

As reported in 0e0d0f5#r29318076, there is an apparently needless check for BUILD_SHARED_LIBS regarding private dependencies of static targets.

https://github.com/robotology/ycm/blob/11f45a366fee3e0f35d847c7cfaa601cc5b9687d/modules/InstallBasicPackageFiles.cmake#L559-L570

If I don't set BUILD_SHARED_LIBS (it's either not defined or set to OFF), but all my libraries are SHARED, private dependencies will be handled anyway. And of course, regardless of the value of said variable, STATIC/SHARED options to add_library() override its behavior. Perhaps the inner else clause should get pulled up and no check for BUILD_SHARED_LIBS be performed at all?

In addition, as reported in #148, targets beyond the first one (passed to the FIRST_TARGET option) will be ignored when using the EXPORT option.

[traversaro]

fyi @randaz81, this is the problem that affected you.

[randaz81]

Yes indeed!