[Validator] Allows Swift pods to have a deployment target under iOS 8.0 ...

[XCConfigHelper] Adds $(inherited) to generated OTHER_SWIFT_FLAGS

[Resolver] Shows an informative error message when there is no base specification found for a `:head` dependency.

Closes https://github.com/CocoaPods/CocoaPods/issues/3230.

[Validator] Lint dynamic libs for iOS > 7

Fix static binaries check

Allows the usage of `:head` dependencies even when the most recent published version was a pre-release.

Closes https://github.com/CocoaPods/CocoaPods/issues/3212.

Set FRAMEWORK_SEARCH_PATHS only when needed

We don't need to add $PODS_FRAMEWORK_BUILD_PATH, if there is no pod target, which should be build.
Fixes #3213.

Closes https://github.com/CocoaPods/CocoaPods/issues/2990.

See 122c8d98

Logical expressions are hard.

Set SKIP_INSTALL=YES for all generated targets