cmake: fix for Xcode-only installs

Help cmake to find the files located within the SDK by writing
a CMAE_FIND_ROOT_PATH.txt.
wxmac: fixing compiler clang/llvm issues.

- wx 2.9 needs a patch to compile with clang when using enable-webkit
- wx 2.8 fails with clang → use llvm if MacOS.lion?
- Use new ENV.macosxsdk(version) to set 10.6 target
- Need to pass --with-macosx-version-min to configure
- fails_with :clang unless --devel
- Add path to Python.h for Xcode only install #10510
- removed the --devel (it shadows homebrew's default)
- Actively check for python with --framework

It is not enough to set the env flags, we need to give
configure the MacOS.version, otherwise it may default to 10.5
on a 10.7 system. (This happens on Xcode 4.3 w/o CLT)

We don't want the user to wait for the long build
of wx just to see homebrew printing that you'll need
the framework build of python at the end (caveats).
So we check this early on `def install`.
python: Do not link against system's python lib!

- Fix http://bugs.python.org/issue11445.
This caused various bugs and build failures of certain modules.
- made the dynamic extensions of sqlite optional, since it can lead to not building sqlite at all.
Core change: XCode only install, with CLT or both

Allow XCode without the Command Line Tools to
work with homebrew, so it's not necessary
to register an Apple Dev ID and/or go to the
XCode prefs and download the CLT. Yay!

Further, this commit allows to use the CLT
solely (without the need for XCode).
Saves quite some megs.
(Some furmulae require xcodebuild)

Of course XCode together with the CLT is still
fine and has been tested on 10.7 and 10.6
with Xcode 4 and Xcode 3.

New in utils:
  MacOS.locate method to get the path to a
  binary tool. It issmart enough to look in
  /usr/bin or query "/usr/bin/xcrun" tool.
  As a final resort find the needed tool within
  Xcode Toolchains or in the MacOS.dev_tools_path.
  Note, xcrun has been removed from ENV.rb.

New MacOS.xcode_installed? telling us whether
the Xcode is installed or not.

MacOS.xcode_version may return a version string
that is guessed based on the compiler, so do not
use it in order to check if Xcode is installed.

New MacOS.clt_installed? telling us
whether the CLT are installed or not.

MacOS.xctoolchain_path gives the path of the
Xcode toolchain (compiler and such).

MacOS.macosx_sdk_path(version) returning the path
of the MacOSX SDK.

ENV.macosxsdk function sets all needed lib and include
dirs to CFLAGS, CPPFLAGS, LDFLAGS.

In build.rb it is checked if Xcode >= 4.3 and
xctoolchain path exists. If yes, the new
ENV.macosxsdk(MacOSX.version) is called
to set up the environment and PATH.

--config.rb shows the version number of the CLT and
the path to Xcode.app unless in a standard place.

Only on Lion or above, tell the user about the options,
which are
- Xcode without CLT
- CLT without Xcode
- both (ok, it's not directly stated, but implicit)
So if no Xcode is found and we are on Lion or above,
we don't fail but check for the CLTs now.
For older Macs, the old message that Xcode is needed
and the installer should be run is still displayed.
If the CLT are not found but Xcode is, then we
print out about the experimental status of this setup.

ENV.x11 respects macosx_sdk_path

Core-change: keg_fix_install_names now uses MacOS.locate.