Should provide gmake as an alias to make

[sschuberth]

The DevKit only ships with make.exe, but e.g. rake-compiler gives precedence to gmake. If other tools like Strawberry Perl that provide gmake are in PATH, a build of native extensions would fail as that gmake likely does not understand MSYS paths.

A way to fix this would be to provide an alias / copy of make named gmake. At least rake-compiler would also allow to override the make tool by setting the MAKE environment variable, but that's probably harder to get right.