Disambiguate/Specialize MAKEFLAGS for GNU Make

[guilt]

From what I've been looking at, is have alternate ways to set GNU Make specific MAKEFLAGS? (if two variants of make are installed in a system, such as nmake).

Then it may be possible to set GMAKEFLAGS=j17 so that other commands cannot parse this variable. The old behavior would still be helpful for systems which do not have this problem.