The SRCDIR= directive is optional. (formerly SOURCEDIR=)

      Use it if you do not keep your source code in the directory where
      you run the make script from.

     You may also override the SRCDIR=, which is active across all
     sections with individual directives for each section.

     So, you can do this:

     SRCDIR=SRC\

     SRCDIR2=\OTHER\SRC