Add configure options to point the build at Apple libc and XNU source, required to find private headers on Mac OS X.
Add configure options to point the build at Apple libc and XNU source,
required to find private headers on Mac OS X.

Use AC_ARG_ENABLE instead of AC_ARG_WITH for functional frobs, such as
legacy API, libdispatch_init constructor tagging, Apple-specific TSD
and semaphore optimizations, as well as new settings such as the use of
crashreporter_info.

Locate MIG on Mac OS X, as we need it to build protocols.def.

Use pthread_workqueue.h to decide whether to define
HAVE_PTHREAD_WORKQUEUES.  We may want to offer a --disable frob for this
on Mac OS X.



git-svn-id: fd4a176c55b201366617f1b4a14a5ec6630ef30e@24 5710d607-3af0-45f8-8f96-4508d4f60227

jcbertin/libdispatch,evaautomation/libdispatch,PSPDFKit-labs/libdispatch,nickhutchinson/libdispatch,nickhutchinson/libdispatch,PSPDFKit-labs/libdispatch,mir-ror/libdispatch,SproutOrc/libdispatch,Chilledheart/libdispatch,nickhutchinson/libdispatch,jcbertin/libdispatch,nickhutchinson/libdispatch,mir-ror/libdispatch,jcbertin/libdispatch,evaautomation/libdispatch,rpaulo/libdispatch-freebsd,PSPDFKit-labs/libdispatch,evaautomation/libdispatch,PSPDFKit-labs/libdispatch,evaautomation/libdispatch,SproutOrc/libdispatch,mir-ror/libdispatch,JackieXie168/libdispatch,SproutOrc/libdispatch,Chilledheart/libdispatch,JackieXie168/libdispatch,JackieXie168/libdispatch,rpaulo/libdispatch-freebsd,Chilledheart/libdispatch,rpaulo/libdispatch-freebsd,JackieXie168/libdispatch,SproutOrc/libdispatch,JackieXie168/libdispatch