Setting up jhbuild
To set up jhbuild, follow the basic installation instructions from the jhbuild manual. After you have installed jhbuild, you should copy the sample jhbuild configuration file into your home directory by executing the following command from the jhbuild directory:
$ cp examples/sample.jhbuildrc ~/.jhbuildrc
The gtkmm module is defined in the gnome-suites-core-deps-3.x.modules moduleset, so edit your .jhbuildrc file and set your moduleset setting to the latest version e.g. like so:
moduleset = 'gnome-suites-core-deps-3.12'
After setting the correct moduleset, you need to tell jhbuild which module or modules to build. To build gtkmm and all of its dependencies, set modules like so:
modules = [ 'gtkmm' ]
You can build several modules by setting the modules variable to a meta-package, e.g. meta-gnome-core, or listing more than one module name. The modules variable specifies which modules will be built when you don't explicitly specify anything on the command line. You can always build a different moduleset later by specifying it on the commandline (e.g. jhbuild build gtkmm).
By default, jhbuild's configuration is configured to install all software built with jhbuild under the /opt/gnome prefix. You can choose a different prefix, but it is recommended that you keep this prefix different from other software that you've installed (don't set it to /usr!) If you've followed the jhbuild instructions then this prefix belongs to your user, so you don't need to run jhbuild as root.
When you downloaded jhbuild from the git repository, you got a number of .modules files, specifying dependencies between modules. By default jhbuild does not use the downloaded versions of these files, but reads the latest versions in the git repository. This is usually what you want. If you don't want it, use the use_local_modulesets variable in .jhbuildrc.