make.py (and the pymake modules that support it) are an implementation of the make tool which are mostly compatible with makefiles written for GNU make. http://benjamin.smedbergs.us/pymake/
You need to have have MozillaBuild installed. Additionally, you need to ensure your mingw shell is instructed to |export SHELL| (exactly that) before you try to run pymake.
(Temporarily, you also need to apply the patches in https://bugzilla.mozilla.org/show_bug.cgi?id=520359 to get pymake working.)
Follow the build instructions, except for the "Build and install" section. Switch to the root of the source directory, then run autoconf-2.13. Change to js/src and run autoconf-2.13 again. Change to your objdir and run path_to_srcdir/configure. Running python -O path_to_srcdir/build/pymake/make.py starts the build. If you hadn't specified mk_add_options MOZ_MAKE_FLAGS in your mozconfig, you can simply append the flags to the aforementioned Python command, e.g. -j2 runs two make jobs in parallel. Using a -j value equal to the number of CPU cores visible to the OS is recommended.