Building world with clang

Hi,

Since clang has gone into the tree, there has been an effort to get head
in a state where world can optionally be built with it.  A number of
changes required for this have already been committed, mainly thanks to
Ed Schouten, Roman Divacky and Rui Paulo.  Most of these changes were
adapted from the clangbsd project branch.

There are several other changes in the queue, pending review and/or
further enhancement, but I would like to solicit your comments about one
particular set: the changes required to let buildworld use clang as the
compiler.

Probably the most logical way to specify that you want world built with
clang, is to put CC=clang and CXX=clang++ in /etc/src.conf.  Any
necessary modifications to Makefile.inc1 or bsd.*.mk can then be put
between conditionals like:

.if ${CC:T:Mclang} == "clang"
[...stuff specific to clang...]
.endif

so nothing will change for non-clang builds.

Now, for building clang as the bootstrap compiler, there are basically
two methods to make it use the correct headers, C startup objects, and
libraries from the object tree (${WORLDTMP}):

1) The "isysroot" method: build a regular version of clang, and make
   sure WMAKEENV contains something like:

   CC="${CC} -isysroot ${WORLDTMP} -B${WORLDTMP}/usr/lib/ \
                                   -L${WORLDTMP}/usr/lib/"

2) The "tools-prefix" method: build a special version of clang, that
   has its default search paths for headers, startup objects and
   libraries modified, to look for everything under ${WORLDTMP}.

Method 1) is used in the clangbsd project branch.

Method 2) is similar to what is used for building the in-tree gcc as
the bootstrap compiler.  During the cross-tools stage, TOOLS_PREFIX is
defined to point at ${WORLDTMP}, and the bootstrap gcc's built-in
search paths get prefixed with it.

An advantage of method 1) is that it does not require any modifications
to the compiler, and basically just a few extra command line arguments.
The same method could probably even be made to work for gcc.

However, a disadvantage is that the built-in search paths of the
bootstrap compiler are not entirely disabled by using the -isysroot, -B
and -L flags, so there is still a chance that headers, objects or
libraries outside of ${WORLDTMP} will be picked up during the world
stage.

An advantage of method 2) is that you can be 100% sure all built-in
search paths of the bootstrap compiler point to directories under
${WORLDTMP}.  Of course, a disadvantage is that you have to make some
modifications to the compiler source itself.

I would like to hear your opinions about which method is preferred, or
if there may be another good way to solve the bootstrap compiler issue.

I have also attached two patches to this mail, which can be applied to
head, to show the exact set of changes required for each method.