parallel destroot problematic

[Ryan Schmidt]

"use_parallel_build yes" does more than just run the build phase in  
parallel. It also runs the destroot phase in parallel. This has  
caused issues for libxml2 and mysql5-devel; see:

http://trac.macports.org/ticket/15295

http://trac.macports.org/changeset/36613

Should we change MacPorts to only add the -j argument to the make  
command in the build phase, and not do so in the destroot phase? The  
point of running the build in parallel is to make use of additional  
CPU cores, but the destroot phase is concerned only with copying  
files on disk. Extra CPU cores won't speed that up anyway.