Speedup of documentation generation

[albert-github]

The current ITK documentation generation runs with the, default, setting:

NUM_PROC_THREADS       = 1

It would be beneficial to use NUM_PROC_THREADS with 0 or at least an higher number of threads.

From the doxygen documentation:

The NUM_PROC_THREADS specifies the number of threads Doxygen is allowed to use during processing. When set to 0 Doxygen will based this on the number of cores available in the system. You can set it explicitly to a value larger than 0 to get more control over the balance between CPU load and processing speed. At this moment only the input processing can be done using multiple threads. Since this is still an experimental feature the default is set to 1, which effectively disables parallel processing. Please report any issues you encounter. Generating dot graphs in parallel is controlled by the DOT_NUM_THREADS setting.

Minimum value: 0, maximum value: 32, default value: 1.