Saving time with CPU_COUNT

The most time consuming task for building the documentation is running LilyPond to build images of music, and there cannot be several simultaneously running lilypond-book instances, so the -j make option does not significantly speed up the build process. To help speed it up, the makefile variable CPU_COUNT may be set in local.make or on the command line to the number of .ly files that LilyPond should process simultaneously, e.g., on a dual core machine:

make -j2 CPU_COUNT=2 doc

The recommended value of CPU_COUNT is the number of cores. If the build runs into out-of-memory problems, use a lower number.