What is the proper way to include multiple entries in Custom.m4?

The Custom.m4.example file only includes one custom dissector. I thought that this would work:

m4_define([_CUSTOM_AC_OUTPUT_], [plugins/foo/Makefile, plugins/bar/Makefile, plugins/baz/Makefile]

but this causes aclocal to exit with an error and halt the build during ./autogen.sh.

aclocal -I ./aclocal-fallback
/usr/bin/m4:configure.ac:2976: Warning: excess arguments to builtin `define' ignored
/usr/bin/m4:configure.ac:2976: Warning: excess arguments to builtin `ifdef' ignored
autom4te: /usr/bin/m4 failed with exit status: 1
aclocal: error: echo failed with exit status: 1

Clearly I have the syntax wrong. What is the correct construct to include multiple custom dissectors in Custom.m4?