Add support for conditionals inside USE dependencies. This is useful

Add support for conditionals inside USE dependencies. This is useful
for cases in which the package satisfying a dependency should have
identical or opposite flag state relative to the parent package.

Conditionals are evaluated as follows:

   parent state   conditional   result
     x              [x?]          [x]
    -x              [x?]         [-x]
     x             [-x?]         [-x]
    -x             [-x?]          [x]

svn path=/main/trunk/; revision=10396