SR-438: 'make distcheck' fails for libdispatch on Linux
SR-438: 'make distcheck' fails for libdispatch on Linux

Small fixes to (a) use proper paths to libpwq when building
out-of-tree and (b) include tests/linux_port.h in list of
test sources (needed for distcheck).  With these changes
'make distcheck' works as expected.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch