During the exp-run in bug 208158, it was found that x11-toolkits/wxgtk30
gives errors with libc++ 3.8.0:

./src/stc/scintilla/src/Editor.cxx:5844:6: error: call to 'abs' is ambiguous

if (abs(pt1.x - pt2.x) > 3)
    ^~~

This is because pt1.x and pt2.x are float values, but <cmath> is not
included, and abs() for float values comes from that header. Fix it by
including <cmath>.

Approved by: portmaster@bsdforge.com (maintainer)
Approved by: vlad-fbsd@acheronmedia.com (maintainer)
PR: 209694
MFH: 2016Q2