`make index' should use MAKE_JOBS_NUMBER / -jNN value

[wosch]

Summary

I tried to build the INDEXFILE. The `make index' command uses all available CPUs. However, I cannot limit the number of CPUs with
`make -j3 index' to 3, or by setting the variable MAKE_JOBS_NUMBER=3

This patch allow use to specify the number of jobs with the command line parameter -j or the variable MAKE_JOBS_NUMBER

make -s PORTSDIR=$(pwd) INDEX_ECHO_1ST=true MAKE_JOBS_NUMBER=3 index
make -s PORTSDIR=$(pwd) INDEX_ECHO_1ST=true -j3 index

See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=288349

[wosch]

add forgotten check for .MAKE.JOBS