Fix GNU make detection and usage.

Fix GNU make detection and usage.

BSD make errors out since the Makefiles use GNU make specific code.
However, on BSD systems, 'make' is BSD make and not GNU make.

make(1) does set ${MAKE} to argv[0], so use this to pass the correct
make executable name down.

Also, prefer gmake to make, if it exists, since we want GNU make.

Signed-off-by: Thomas Klausner <tk@giga.or.at>
Tested-by: Gabriel Filion <lelutin@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>