https://bugs.webkit.org/show_bug.cgi?id=114925

Reviewed by Geoffrey Garen.

This enables building LLVM along with WebKit, so that build-jsc and build-webkit
will also optionally build LLVM and quickly symlink LLVM's built products into
the right places.

Most WebKit and JSC hackers will want to rely on the checked-in already-built
versions of LLVM in WebKitLibraries. But developing both systems in tandem is an
increasingly common use-case for me, and it may become a common use case for a
handful of others. Currently, this is really painful: you first have to build
LLVM, then you have to export-llvm-build (which takes a while), and then you
have to make sure that your LLVM_LIBRARY_PACKAGE and LLVM_INCLUDE_PACKAGE
variables are set to point to the thing you exported. The whole process loses
track of dependencies very quickly: making a tiny change in LLVM requires
packaging, and then unpackaging, a large number of potentially large headers and
static libraries. Not only is this slow but it then causes the WebKit build
system to rebuild anything that transitively includes any LLVM header, which is
now quite a few files. While this sort of use pattern is still worthwhile if
you're trying to package a binary drop and test it, it's not great if you're
just trying to do experimental development that involves making small changes
in both trees.

This change fixes this use case while keeping the old use cases intact. You can
do tandem development using one of two modes:

Your own LLVM directory: just set LLVM_SOURCE_PATH to the *absolute* path of
the LLVM directory you're using. Once this is done, any invocation of a WebKit
build via build-jsc or build-webkit will also build LLVM, and then quickly
symlink things into place without perturbing dependency tracking.

Internal LLVM directory: if you check out llvm into a directory called 'llvm'
right off of the WebKit source tree, then the build system will automatically
use this.

Here's how this takes care of dependencies:

Headers: the include/llvm and include/llvm-c directories are symlinked into
$productsDir/usr/local/include. And then everything just works.

Libraries: the build system detects, by reading LLVM's Makefile.config, which
mode LLVM is built in (like Release+Asserts or Debug+Asserts) and symlinks
the .a files into $productsDir/<thingy>. It will ranlib those libraries only
if they have changed, by checking both the modification time and also whether
the last time we had a symlink, that symlink was from the same directory.
This helps if you switch to an *older* LLVM build (using LLVM_SOURCE_PATH)
but that build wasn't yet ranlib'd.

One problem that this does not yet solve is that xcodebuild will not relink
JavaScriptCore if the only thing that changed was the libraries. I will work
on this problem separately: https://bugs.webkit.org/show_bug.cgi?id=114926.

* Scripts/copy-webkitlibraries-to-product-directory:
(unpackIfNecessary):
(fileContains):
(fileContentsEquals):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153125 268f45cc-cd09-0410-ab3c-d52691b4dbfc