[IMP-dev] Compiling IMP and slow machines

If you computer has been getting unusually slow while building IMP, that is a side effect of the change I made a few days back (merging C++ .cpp files in a module before compilation). The "problem" is that it makes compilation significantly faster (1000s vs 2000s for a clean build of IMP on my desktop), making the total build mostly limited by disk I/O. The result is that one often ends up with all the compilation processes writing as fast as possible to your hard drive, which can make the gui sluggish. One can trade off slower compilation for more ability to do other things while it is compiling by either turning off the merge of .cpp files (percppcompilation="yes") or compiling with fewer processes (eg -j 2 instead of -j 4).