PREFIX is not honored for dependencies

[Steve Kargl]

As root, I did

mkdir -p /usr/opt
ln -s /usr/opt /opt
cd /usr/ports/editors/libreoffice
make PREFIX=/opt/

The above builds and installs dependencies along the
way under the /opt/ heirarchy.  The first few are

cppunit-1.12.1
mdbtools-0.5_14 
boost-libs-1.45.0_1

The build finally dies with qt4-moc, because it does not
honor PREFIX.  I've attempted to work around the QT4 issue
by patching its Makefile.  So, I restart a build

make clean
make PREFIX=/opt/

This eventually dies with 

===>  Installing for cppunit-1.12.1
===>   Generating temporary packing list
===>  Checking if devel/cppunit already installed
===>   cppunit-1.12.1 is already installed
      You may wish to ``make deinstall'' and install this port again
      by ``make reinstall'' to upgrade it properly.
      If you really wish to overwrite the old port of devel/cppunit
      without deleting it first, set the variable "FORCE_PKG_REGISTER"
      in your environment or the "make install" command line.

Of course, cppunit-1.12.1 is installed!  The initial build
in libreoffice installed it before qt4-moc crooked.   

Please fix.

-- 
Steve