Do not generate targets for Pods without sources

If a PodTarget should not be build:
- Do not install a target
- Skip the non-existing native targets in Installer
- Do not add linker flags to XCConfig
Added changelog entry.
Specs for vendored libraries/frameworks.
Improve naming to `should_build?`
Simplify `should_build?`
[Spec] Expect what is documented to happen

Thanks @segiddins.
Merge pull request #2918 from CocoaPods/dont-generate-targets-if-no-source-files

Do not generate targets for Pods without sources