[qepcad-B.1.71] ar crvU sfexto.a BPOLSETS.o
[qepcad-B.1.71] /Library/Developer/CommandLineTools/usr/bin/ar: illegal option -- U

A few more linux-specific things need to be addressed to make it compile.

Also, QEPCAD B.1.72 is out:

• https://www.usna.edu/Users/cs/wcbrown/qepcad/INSTALL/qepcad-B.1.72.tar.gz
  and Fedora has a number of valuable patches that we should use:
• https://src.fedoraproject.org/rpms/qepcad-B/tree/master

Also fails on ubuntu-bionic (https://github.com/mkoeppe/sage/runs/430688625?check_suite_focus=true)
and ubuntu-bionic-i386 (https://github.com/mkoeppe/sage/runs/430689089)

ubuntu-trusty (https://github.com/mkoeppe/sage/runs/434148305?check_suite_focus=true):

qepcad.a(MAIN.o): In function `sendSignalAfterInterval':
/sage/local/var/tmp/sage/build/qepcad-B.1.72/src/source/main/MAIN.c:93: undefined reference to `timer_create'
/sage/local/var/tmp/sage/build/qepcad-B.1.72/src/source/main/MAIN.c:102: undefined reference to `timer_settime'

Also Fedora 26 (https://github.com/mkoeppe/sage/runs/430688790?check_suite_focus=true, https://github.com/mkoeppe/sage/runs/430688804?check_suite_focus=true#step:4:10),
Fedora 28 (https://github.com/mkoeppe/sage/runs/430688823?check_suite_focus=true)

This ticket fixes these problems.
As seen here: https://github.com/mkoeppe/sage/actions/runs/40514876

CC: @dimpase @videlec @tscrim

Component: packages: experimental

Author: Matthias Koeppe

Branch/Commit: 56d30c2

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/28388