m4 1.4ppre2 doesn't link in Linux

[lars brinkhoff]

Linking m4 version 1.4ppre2 fails in Linux.  My quick'n'dirty
solution was to add libdl.so in src/Makefile, like so:

        m4_LDADD = /usr/lib/libdl.so $(top_srcdir)/lib/libm4.la 
                  ^^^^^^^^^^^^^^^^^^