mail-client/neomutt: fix configure with ccache

mail-client/neomutt: fix configure with ccache

If you have dev-util/ccache installed but not FEATURES="ccache", neomutt's
configure process will detect and try to use it. You can override this
by setting an env var, CCACHE, to 'none' (and only that, see line 1132
or so of neomutt's source file auto.def).

Without this change, you will get the following sort of error:

>>> Configuring source in /tmp/portage/mail-client/neomutt-20180716/work/neomutt-neomutt-20180716 ...
./configure --prefix=/usr --build=x86_64-pc-linux-gnu ....
Host System...x86_64-pc-linux-gnu
Note: defaultprefix is deprecated. Use options-defaults to set default options
Build System...x86_64-pc-linux-gnu
C compiler...ccache x86_64-pc-linux-gnu-gcc -O2 -pipe -march=znver1
C++ compiler...ccache x86_64-pc-linux-gnu-c++ -O2 -pipe -march=znver1
Build C compiler...cc
 * ACCESS DENIED:  utimes:       /var/cache/ccache

Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>
Signed-off-by: Nicolas Bock <nicolasbock@gentoo.org>