Makefile: Unexport LC_ALL instead of clearing it

Apparently not all versions of glibc and utilities treat an empty
LC_ALL as nonexistent, causing error messages to be garbled.  Instead,
explicitly unexport it from the environment.

Reported-and-tested-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <4B2AC394.4030108@redhat.com>
Cc: Michal Marek <mmarek@sues.cz>
Cc: Roland Dreier <rdreier@cisco.com>
Cc: Sam Ravnborg <sam@ravnborg.org>