[v3] remove serial from two .m4 autoconf functions

[Benjamin Kosnik]

I don't know what these do, sadly. Autoconf 2.61 warns:

acinclude.m4:2551: the serial number must appear before any macro
definition 
linkage.m4:538: the serial number must appear before any macro
definition 

Removing them seems to have no impact, so I'd like to do this.

-benjamin
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