Take a more radical approach to disabling colours: pretend we don't

Take a more radical approach to disabling colours: pretend we don't
HAVE_TERMINFO in our local config.h.  Some of the lower levels of LLVM
don't seem to respect our default setting.

As a result, delete the -ltermlib dependency.

Note that this breaks -fcolor-diagnostics=auto.  However, fixing the whole
codebase is out of scope for now ...

Prompted by an instance found in the AsmParser by deraadt@

ok kettenis@ patrick@