Issue Description:

cflags and libs obtained by parsing .pc are space separated files but paths can also contain spaces in them. Right now they're being split blindly on encounter of a space, this should be handled. For eg: a flag might be :
"-I/usr/local/Cellar/gtk+3/3.18.9/include/gtk-3.0 -I/usr/local/Cellar/glib/2.46.2/include/gio-unix-2.0/" which is converted into string array :
["-I/usr/local/Cellar/gtk+3/3.18.9/include/gtk-3.0", "-I/usr/local/Cellar/glib/2.46.2/include/gio-unix-2.0/"]
If the flag is:
"-I/usr/local/Cel\ lar/gtk+3/3.18.9/include/gtk-3.0 -I/usr/local/Cellar/glib/2.46.2/include/gio-unix-2.0/"
it would be converted to:
["-I/usr/local/Cel", "lar/gtk+3/3.18.9/include/gtk-3.0", "-I/usr/local/Cellar/glib/2.46.2/include/gio-unix-2.0/"]
The correct output should be :
["-I/usr/local/Cel\ lar/gtk+3/3.18.9/include/gtk-3.0", "-I/usr/local/Cellar/glib/2.46.2/include/gio-unix-2.0/"]

Related FIXME in code: https://github.com/apple/swift-package-manager/blob/master/Sources/Build/PkgConfig.swift#L40

What you'll learn:
The problem is pretty easy to solve but it'll get one started on setting up the swiftpm development environment as well writing and running swiftpm tests.