Change:

num_jobs: $(echo "`grep -c ^processor /proc/cpuinfo / 2`" | bc)

to:

num_jobs: $(echo "`grep -c ^processor /proc/cpuinfo` / 2" | bc)

Below is an example of running the original showing the error and correction:

$ echo "`grep -c ^processor /proc/cpuinfo / 2`" | bc
grep: /: Is a directory
grep: 2: No such file or directory
(standard_in) 1: syntax error
(standard_in) 1: illegal character: :
(standard_in) 1: syntax error
(standard_in) 2: illegal character: :

$ echo "`grep -c ^processor /proc/cpuinfo` / 2" | bc
4