[LinuxCNC/linuxcnc Issue#570] Undefined reference to negate inline function

Here are the steps I follow to reproduce the issue:

1. create a project using linuxcnc library;
2. try to compile;

This is what I expected to happen:

Compile without errors.

This is what happened instead:

Compile with linker errors: undefined reference to function negate, file blendmath.c, line 85.

Information about my hardware and software:

* I am using this Linux Mint version 19.1;
* I am running a simple Qt application built with G++ 7.3.0;
* I am using the latest LinuxCNC version;