slatec.pd adds trailing underscores when needed to link with
FORTRAN function, based on the presence of a file
"f77_underscore" created by Makefile.PL. That is for every function
except "polfit" and "dpolfit" which gets an underscore whatever.
The attached patch gets the Slatec extension to compile fine with
IBM xlf FORTRAN compiler on MacOS X (and pass the tests fine