autogen: add shortcut for building with clang

For a while I have been cleaning up warnings when building with clang.
There are currently only two sources of warnings left: Wcast-align and Wgnu.
I am not convinced that fixing up those up is feasible so I run with them
disabled to spot regressions. E.g. clang is a bit more strict wrt to unused
variables with the cleanup attribute and I have fixed a number of those since.

Like the other options in autogen.sh I have a shortcut for clang as well. I use
'l' for llvm.