Builds can depend on non-Gradle-built libraries

We need to support a way to declare dependencies on libraries that are not built by Gradle or do not need to be built at all (e.g., checked in binaries). A build author can do something "low level" by adding the appropriate -I or -L arguments as necessary, but this complicates how we would do up-to-date checks and know if it's safe to cache certain compilation tasks.

As a build author, I would like to be able to declare my dependencies on libraries (#165 covers system libraries) like any other Gradle dependency.

In the software model, we have the concept of a prebuilt library, but it has some downsides.

When you're configuring the headers of a prebuilt binary you're actually configuring the headers of the enclosing library, so something like

model {
    repositories {
        libs(PrebuiltLibraries) {
            util {
                binaries.withType(StaticLibraryBinary) {
                    headers.srcDir "3rd-party-lib/util/src/util/headers/$targetPlatform"
                }
            }
        }
    }
}

...looks like you're adding 3rd-party-lib/util/src/util/headers/$targetPlatform as an include path that's different for each target platform, but you're actually adding all platforms (order of include paths depends on the order the binaries are iterated over).

Start by reviewing the software model's prebuilt library configuration and the dependency declarations described in the issue. Define how externally built or checked-in libraries should be declared, including target-specific headers and cache or up-to-date behavior; done means the model can represent these dependencies without mixing headers from other platforms.