Specify C[++] compiler in LLVM build.

Sometimes LLVM apparently wants to link itself with cc rather than c++. This doesn't happen if we explicitly set CMAKE_CXX_COMPILER.
Recommend the use of debug builds.
Drop -fcolor-diagnostics.

This is only really for people who are hacking on TESLA, it's cosmetic and it might confuse end users.
We have more targets now.
For consistency, 'TESLA' -> 'tesla'.
We only need 'dot', not the GraphViz libraries.