Fix some minor build issues.

1. Configure script will now explicitly check, that llvm is found
   and give a sane error message otherwise

2. Since we ship Makefile and configure, then we should assume,
   that users will not read documentation and just run make or
   ./configure or both. Now, scripts will auto-magically figure out
   what is happening and run oasis by themselves.

3. Everybody reports the warning from oasis as errors. It is really
   frustrating. Many even stop after seeing the warning. This PR will
   reduce the amount of warnings. First of all oasis is called inside
   configure script with -quiet option. Next, the proper environment
   variables are set, so that ocamlfind eschew to emit extra warnings.

4. Many systems by default name llvm-config just as llvm-config, and do
   not provide files with explicit version numbers, unlike ubuntu does.
   This PR addresses this issue, using by default `llvm-config` on
   linux, and `llvm-config-mp-3.4` on macosx. Adding explicit version
   requirements with command line argument `--with-llvm-version=<ver>`
   will append the <ver> to the filename.