Improve detection on FindLibs

Archlinux's headers for c-client/imap are a little odd, missing the auth_gss symbol, and thus failing to compile. Also, FindLibDwarf should also look for `libdwarf.h`, since on Archlinux, there is `/usr/include/dwarf.h` and `/usr/include/libdwarf/{,lib}dwarf .h` it gets confused and doesn't set the includes correctly.