By using common/gcc-base-unix.conf and common/g++-unix.conf most of the
mkspec details can go, leaving us with a minimized qmake.conf.

Change the default X11 location to point to /usr/X11R7 after NetBSD
switched to X.org in Release 5.0 by default.

Also change the default location for addon software on NetBSD, it uses
pkgsrc as its ports system which installs to /usr/pkg by default (the
default value for $LOCALBASE of pkgsrc on NetBSD), see
https://www.netbsd.org/docs/pkgsrc/using.html

 chapter 4.1.

These default values mirror the current state of NetBSD usage to get
the prerequisites to compile Qt with a minimum amount of configure
parameters.

Tested with NetBSD 7.0 using gcc 4.8.4-nb2 (system compiler)

Change-Id: Ic50757af9f070c0383c7356302e7e7686f031740
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>