Better LLVM detection in the vprc binary

[griwes]

Currently, vprc is hardcoded to search for llvm-config. It should try to detect LLVM more widely, since sometimes llvm-config is missing; we should also look for llvm-config-[0-9]+.

Additionally, the process launch for LLVM binaries should be wrapped in try-catch - the current method of error detection from running those processes is just to check for an exit code, but it seems that Boost.Process throws an exception on launch failure. Oops.