Overhauled llvm/clang docs builds. Closes PR6613.

NOTE: 2nd part changeset for cfe trunk to follow.

*** PRE-PATCH ISSUES ADDRESSED

- clang api docs fail build from objdir
- clang/llvm api docs collide in install PREFIX/
- clang/llvm main docs collide in install
- clang/llvm main docs have full of hard coded destination
  assumptions and make use of absolute root in static html files;
  namely CommandGuide tools hard codes a website destination
  for cross references and some html cross references assume
  website root paths

*** IMPROVEMENTS

- bumped Doxygen from 1.4.x -> 1.6.3
- splits llvm/clang docs into 'main' and 'api' (doxygen) build trees
- provide consistent, reliable doc builds for both main+api docs
- support buid vs. install vs. website intentions
- support objdir builds
- document targets with 'make help'
- correct clean and uninstall operations
- use recursive dir delete only where absolutely necessary
- added call function fn.RMRF which safeguards against botched 'rm -rf';
  if any target (or any variable is evaluated) which attempts
  to remove any dirs which match a hard-coded 'safelist', a verbose
  error will be printed and make will error-stop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103213 91177308-0d34-0410-b5e6-96231b3b80d8

docs/HistoricalNotes/2003-01-23-CygwinNotes.txt

@@ -1,28 +0,0 @@
-Date: Mon, 20 Jan 2003 00:00:28 -0600
-From: Brian R. Gaeke <gaeke@uiuc.edu>
-Subject: windows vs. llvm
-
-If you're interested, here are some of the major problems compiling LLVM
-under Cygwin and/or Mingw.
-
-1. Cygwin doesn't have <inttypes.h> or <stdint.h>, so all the INT*_MAX
-   symbols and standard int*_t types are off in limbo somewhere. Mingw has
-   <stdint.h>, but Cygwin doesn't like it.
-
-2. Mingw doesn't have <dlfcn.h> (because Windows doesn't have it.)
-
-3. SA_SIGINFO and friends are not around; only signal() seems to work.
-
-4. Relink, aka ld -r, doesn't work (probably an ld bug); you need
-   DONT_BUILD_RELINKED. This breaks all the tools makefiles; you just need to
-   change them to have .a's.
-
-5. There isn't a <values.h>.
-
-6. There isn't a mallinfo() (or, at least, it's documented, but it doesn't seem
-   to link).
-
-7. The version of Bison that cygwin (and newer Linux versions) comes with
-   does not like = signs in rules. Burg's gram.yc source file uses them. I think
-   you can just take them out.
-