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Issue Description:

Hi all,
It was recommended I create a ticket, so here it is.

As a test, I just wanted to import the CCairo system module. That and everything else mentioned here will be in a zip archive below. So I created a directory named Sources, and two folders named example and CCairo. I had put a cairo.so.2xxxx file in the "Link: " section of the module.modulemap file, and when run, I had a main.swift file that only had "import CCairo" inside of it.

When ran, the thing spat this:

Linking Executable: .build/debug/example
/usr/bin/ld: cannot find -l/usr/lib/x86_64-linux-gnu/libcairo.so
clang: error: linker command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: link command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: build had 1 command failures
swift-build: exit(1): ["/home/mgage/Downloads/swift-2.2-SNAPSHOT-2015-12-01-b-ubuntu15.10/usr/bin/swift-build-tool", "-f", "/home/mgage/Sources/example/.build/debug/example.o/llbuild.yaml"]

I went on the mailing list with the above machine-jabber, and was told it looks like the linker itself had a bug. I'm using the latest (2.2) and had added the thing to my path so I could use it from anywhere. Also, I'm on Ubuntu GNOME 15.04, so to take care of that possible question. Also, I'm new to Swift, but I thought it must be a big deal given that Apple hardly ever does anything like this.