Various fixes to the build

First: libdwarf. libdwarf seems to have no problem making breaking
changes and provides no easy way to even test the version you're
building against. I have added two tests in `FindLibDwarf.cmake` to
detect a change to function type signature and to detect the removal
of `dwarf_producer_init_c`. Hopefully this will work on all systems.

The recent change to use more system libraries broke the build of the
`gen-*` bootstrap utilities, as the targets were not updated to use
the system `double-conversion` library and instead attempted to use
the (unbuilt) local one.

Commit 83cbca11c1 made linking to libpam happen only on OSX. This
breaks the build on ArchLinux and openSUSE at least. I reverted this
change given that detecting whether or not you need to link to it is
quite difficult.
Why are you depending on elf now? What if you have dwarf without elf? Should that be in the if statement?