explicit aspect activation

ag++ is now called with a list of currently active aspect headers
(ag++ -a aspect1.ah -a aspect2.ah ...).  This resolves several problems at
once:
 -  Build directories may be positioned arbitrarily now, they need not be
    a subdirectory of the project anymore.
 -  Multiple build directories can coexist within the project tree.  Before
    this commit, the generated instantiate-*.ah aspect headers disturbed
    neighboring build trees.
 -  Due to this, the regression test should be runnable much more easily
    now.
 -  The build time was reduced by an average of about 10%.

git-svn-id: https://www4.informatik.uni-erlangen.de/i4svn/danceos/trunk/devel/fail@1753 8c4709b5-6ec9-48aa-a5cd-a96041d1645a