Markdown compile documentation

The compile instructions are now in markdown format. I have also added instructions for fetching LLVM, so the steps can be run without additional googling.
Text format remains, but should probably be removed if we keep markdown.

Other stuff:
* I had some issues with ``inline`` being already being defined. This is fixed by adding an ``#undef`` before defining it in ``osdep.h``.
* You can now use a different compiler, without changing ``build.sh``.
* Added missing linker flags to ``build.sh``.
* ``.gitignore`` now hides the llvm/distorm dirs.