Checkpoint. Works.
Checkpoint.
Updating the script to work with current git versions.

Main difference is that LLVM 3.4 also builds libc++ if found in the
projects directory so we don't need to build that manually anymore.
That however changes the build order a bit.

Also unifying the three build stages to reuse the same code with
different options.

Also includes some general cleanup and renaming of the options
selecting what to build.

The three build stage
Setting versions to LLVM 3.4 release branches (where available).
Make build more configurable via cmd line options.

The script now uses the bash getops builtin to parse its command line.
Add flag to specify "make -j" parallelism.
Markdownify README.
Small grammar fix.
More markdownifications.
Create PREFIX on install if non-existant.
Small fixes.
Correctly install shared libraries.
Fall back to a system compiler if $CXX is not set.
Honor $CC and $CXX.

For stage1 builds we have to rely on the system-provided compiler and default
to the Clang toolchain. However, users now can still override the initial
compiler by providing $CC and $CXX.
Enhancement to fix introduced in 5a6d1a08fc.
Add option to start at a specific stage.
Find combination that successfully builds stage1.
Fix stage2 and stage3 builds by linking properly.
Do not compromise Mac OS library export.
Adding a patch that current libc++ release_34 requires to compile with some GCC versions.