Add dynamic make tool detection

[fabiankrack]

On FreeBSD the GNU make binary is called 'gmake'. The native
BSD make cannot be used due to syntax incompatibilities.

[sferik]

What are the specific syntax incompatibilities between BSD make and GNU make? I'd rather we resolve those, if possible. Is there a common syntax that will work in both?

[sferik]

Just noticed this was related to #239 and #266. We should fix this issue, one way or the other. It has dragged on for too long.

@sstephenson @jeremy: What's your preferred approach?

[sferik]

As I understand it, the possible solutions are:

1. Dynamic make selection (this patch)
2. Dynamic MAKE_OPTS selection (This fixes issue #239 related to building Ruby on FreeBSD systems #266)
3. Unify MAKE_OPTS to work with both GNU make and BSD make

In my mind, option 3 would be most optimal/least error prone, but I don't have a good environment to develop and test such a path. Can someone recommend a BSD Vagrant box that will reproduce this issue?

[sstephenson]

I'd prefer if, at the very least, we let you set your own value for $MAKE before falling back to a default value.

[kaspergrubbe]

This can build on both GNU make and BSD make:

MAKE_OPTS="-B -j 2"

I don't know the full implications of the -B-flag. But as I understand it, it does only affect the build process, and makes compiling a little slower.

[sstephenson]

-B has a different meaning in BSD make: http://www.freebsd.org/cgi/man.cgi?query=make&sektion=1

The -j 2 default for $MAKE_OPTS is intended as a helpful optimization for the common case of folks on systems with GNU make and multiple processors. I'd rather explicitly whitelist that configuration than blacklist incompatible configurations.

[kaspergrubbe]

Good catch, I have now read the man page through for both versions of make.

I have made some basic guessing based on the differences of the two programs in my branch: #266

[fabiankrack]

I think the cleanest solution would be to use $MAKE:

#272