Update INSTALL instructions
Update INSTALL instructions

Add description for building libdispatch with Swift API
either as part of building a Swift toolchain from source
using utils/build-script or using a pre-built Swift toolchain.

Update Linux-specific instructions to try to avoid some
common problems people have been reporting.

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