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Issue Description:

The command swift build -X doesn't work for system packages.

For example:

git clone https://github.com/Zewo/OpenSSL
cd OpenSSL
swift build -X

Now open OpenSSL.xcodeproj. Build will fail.
To make it work we need to add some configurations manually:

/usr/local/include in Build Settings > Search Paths > Header Search Paths
/usr/local/lib in Build Settings > Search Paths > Library Search Paths

I think the solution would be to support:

swift build -X -Xcc -I/usr/local/include -Xlinker -L/usr/local/lib

Which would add the correct paths to Xcode configuration automatically.
This also makes sense cause this is the command needed to actually build the module.

swift build -Xcc -I/usr/local/include -Xlinker -L/usr/local/lib

CC: @ddunbar

Edit: I think #235 is needed to reproduce.