Small change needed for link.m4

[John W. Eaton]

The test in link.m4 needs to include <stdio.h> to provide a declaration 
for the rename function.  The test fails if building with GCC's 
-Werror=implicit-function-declaration option.

This was reported as part of a bug with building Octave on Fedora, which 
now uses -Werror=implicit-function-declaration by default.

https://savannah.gnu.org/bugs/?49782

jwe

link-4m-diff.txt
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