Reword the "linux" make target slightly
Reword the "linux" make target slightly

* Ensure that linux_v is always set, no matter what uname says.
* Use the $(shell) make function
* Remove quotes - they seem to not get interpolated for some reason.
* Use mkdir -p to make building work when the gem already has a "man"
  dir.

This fixes #406.

kgrz/zeus,ylansegal/zeus,latortuga/zeus,matrinox/zeus,pelletencate/zeus,latortuga/zeus,kgrz/zeus,matrinox/zeus,burke/zeus,stripe/zeus,stripe/zeus,kgrz/zeus,pelletencate/zeus,Lexxville/zeus,burke/zeus,ylansegal/zeus,burke/zeus,stripe/zeus,Lexxville/zeus,matrinox/zeus,latortuga/zeus,ylansegal/zeus,pelletencate/zeus,Lexxville/zeus