This avoids the need for a confusing line of the form:
DEINSTALL_TEMPLATE+= path/to/INSTALL
in the package Makefile, and actually removes the need to specify it
altogether since by convention, the existence of the DEINSTALL script
is enough to add it to DEINSTALL_TEMPLATE.
* Honor ${PKG_SYSCONFDIR} for the location of the config files
(Muttrc, mime.types).
* Rearrange lines slightly to minimize differences between mutt/Makefile
and mutt-devel/Makefile to simplify updating mutt when the next release
(1.4?) is released.
