[scan-build] Add an option to skip overriding CC and CXX make vars

Autoconf and some other systems tend to add essential compilation
options to CC (e.g. -std=gnu99). When running such an auto-generated
makefile, scan-build does not need to change CC and CXX as they are
already set to use ccc-analyzer by a configure script.

Implement a new option --keep-cc as was proposed in this discussion:
http://lists.llvm.org/pipermail/cfe-dev/2013-September/031832.html

Patch by Paul Fertser!

llvm-svn: 323665