configure: unset harmful environment variables

Apart from CLICOLOR_FORCE and GREP_OPTIONS, there are other variables
that are listed in the Autoconf manual.  While Autoconf neutralizes them
very early, and assumes it does not (yet) run in a shell that has "unset",
QEMU assumes that the user invoked configure under a POSIX shell, and
therefore can simply use "unset" to clear them.

CDPATH is particularly nasty because it messes up "cd ... && pwd".

Reported-by: Juan Quintela <quintela@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -4,9 +4,8 @@
 #
 
 # Unset some variables known to interfere with behavior of common tools,
-# just as autoconf does.
-CLICOLOR_FORCE= GREP_OPTIONS=
-unset CLICOLOR_FORCE GREP_OPTIONS
+# just as autoconf does.  Unlike autoconf, we assume that unset exists.
+unset CLICOLOR_FORCE GREP_OPTIONS BASH_ENV ENV MAIL MAILPATH CDPATH
 
 # Don't allow CCACHE, if present, to use cached results of compile tests!
 export CCACHE_RECACHE=yes