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.
Follow the build instructions, except for the "Build and install" section, then:
- 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 nsprpub/ and run autoconf-2.13 again.
- Create your objdir.
- Finally, 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.