This seems to have been accidentally deleted by commit
a06cd52b4e.
libraries that relate to perl modules and should be linked into a new
perl binary. It writes a corresponding F<perlmain.c> file that
is a plain C file containing all the bootstrap code to make the
+modules associated with the libraries available from within perl.
If the first argument to C<writemain()> is a reference to a scalar it is
used as the filename to open for output. Any other reference is used as
the filehandle to write to. Otherwise output defaults to C<STDOUT>.