Common GNU makefile directory path

You should be able to use the MAKEFILE_LIST variable, like this:

# This must be the first line in Makefile.common
TOP := $(dir $(firstword $(MAKEFILE_LIST)))

From the documentation:

As make reads various makefiles, including any obtained from the MAKEFILES variable, the command line, the default files, or from include directives, their names will be automatically appended to the MAKEFILE_LIST variable. They are added right before make begins to parse them. This means that if the first thing a makefile does is examine the last word in this variable, it will be the name of the current makefile. Once the current makefile has used include, however, the last word will be the just-included makefile.

Try this:

ROOT_DIR := $(dir $(realpath $(lastword $(MAKEFILE_LIST))))

Edit: Be sure to use := instead of = because the latter causes make to use late-binding and MAKEFILE_LIST may have changed due to later includes.