Some systems (notably BSDs) install GNU make as "gmake" - just
invoking "make" and hoping it understands GNU arguments will not
work there. Traditionally, an alternative "make" command can be
selected with the MAKE environment variable.
This changes GNUMakeBuilder (and by extension, AutoconfBuilder)
to invoke the command given in $MAKE and fall back to "make" if
MAKE has not been set.