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.
Add more detailed usage instructions.
Merge remote-tracking branch 'upstream/llvm34' into llvm34

Conflicts:
	install-clang
Remove obsolete patch.

See #4 for details.
Revert "Remove obsolete patch."

This reverts commit 8992a8107eee033a648244252695fecaf97669ec. After discussing
this with Robin further, we'll keep this patch in order to maintain
compatibility with the release34 tag.
Adapt 3.4 versions.