Added support for using a particular version of llvm-link by setting the
LLVM_LINKER environmental variable.
Provide "slightly" more useful error message if the ELF section cannot be found.
Made use of Popen consistent (no namespace qualifier) which is needed for future Popen wrapper.
Added WLLVM_DEBUG environmental variable that will show debug information
when running various tools. Currently this debug information is only what
Popen invocations are. This works by wrapping Popen invocations.