adjust configure.ac for math library

[ringo458]

This is a kind of script auto makefile magic that I have never cared to learn. Anybody know
how to add -lm so that makes proceed with the math library? I have a functioning ps4000 code
under centos 7 but I want to add some floating point processing.

I am pretty sure it takes place in configure.ac and I did make it work in a cheesy way but what is
the correct way? Please