@swift-ci create

On macOS, iOS, etc, the compiler's built-in SDKs are generally relative to the `-isysroot`. But it looks as if the Clang driver does pass `/usr/local/include` to the compiler, and `-L/usr/local/lib` to the linker if those directories exist in the file system. But `swiftc` does not seem to pass `-L/usr/local/lib` to the linker. So if C code, compiled with `clang`, and Swift code, compiled with `swiftc`, are linked together using the `swiftc` driver, that search path will be missing.

It's not ideal to rely on the compiler driver's behavior here, and anyway, the system library can be at a different path. It seems to me that a straightforward option for when a `.pc` file is not being used would be to provide a way for the system library target to impart header and library search paths to any clients by allowing those paths to be specified in the target itself if a `pkg-config` isn't being used.

Actually, it's a bit more complicated. Even though the `clang` driver is passing `-L/usr/local/lib`, the fact that it's also passing `-syslibroot` makes that path get prepended by the path of the SDK. So even if SwiftPM passes `-L/usr/local/lib` to a Clang target, it will end up getting redirected to the SDK. The `swiftc` driver also passes `-syslibroot`, so it wouldn't matter whether SwiftPM passes `-L/usr/local/lib` or not, it would end up getting redirected into the SDK if that path exists there.

Comment by Lane Schwartz (JIRA)

Some new information: https://forums.swift.org/t/swift-package-manager-linker-flags-and-dependencies/36943/26

TL;DR: On macOS 11.2, if there is a pc file in /usr/local/lib/pkgconfig, swift build can find the library, but XCode still can't.