SHELL variable interferes with parallel make

[ceblanton]

FRE compile job logs often show this warning indicating that parallel make is not working properly:

make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

Parallel, or recursive make, ensures that a total number of compile jobs is not exceeded. Communication among the make processes is required to enable this, described here:

https://www.gnu.org/software/make/manual/html_node/Recursion.html#Recursion

Google searches and the make documentation are clear that the child make processes must be called exactly the same or make will bail out of the parallel operation with the warning above.

@underwoo found that the SHELL variable interferes with the parallel make operation, triggering the warning. I reproduced his finding, which is also supported by this documentation on SHELL, which mentions that is not inherited to child make processes. This would seem to explain the problem.

https://www.gnu.org/software/make/manual/html_node/Choosing-the-Shell.html

[ceblanton]

Done, in #44