title: R installation of package `later` fails
body: <p>Software info:</p>
  
  <ul>
  <li>Linux Kernel: 4.14.83-1-MANJARO</li>
  <li>R: 3.5.1</li>
  <li>Rcpp: 1.0.0</li>
  <li>g++ g++ (GCC) 8.2.1 20180831</li>
  </ul>
  
  <p>I'm trying to install the <code>later</code> package from CRAN, but it fails due to a compilation error and I can't figure out whether there is something wrong with my own configuration or the package. The installation stops after the call</p>
  
  <p><code>g++  -I"/usr/include/R/" -DNDEBUG -pthread -DTHREADS_H_SUPPORT=1 -I"/home/karpfen/R-libs/Rcpp/include" -I"/usr/lib/R/library/BH/include" -D_FORTIFY_SOURCE=2  -fopenmp -fpic  -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt  -c timestamp_win32.cpp -o timestamp_win32.o</code></p>
  
  <p>with the error message</p>
  
  <blockquote>
    <p>make: *** No rule to make target '-fopenmp', needed by 'later.so'.  Stop.</p>
  </blockquote>
  
  <p><code>Rcpp</code> and other packages depending on it work as expected. Is there anything that I could be doing wrong here? I tried reinstalling R + packages already, but no changes here.</p>
  
  <p>Edit:</p>
  
  <p>The first few lines of the installation output are</p>
  
  <pre><code>* installing *source* package ‘later’ ...
  ** package ‘later’ successfully unpacked and MD5 sums checked
  Running configure script
  Using CC=gcc
  Using CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt
  Using CPPFLAGS=-D_FORTIFY_SOURCE=2
  C11-style threads.h support detected.
  </code></pre>
  
  <p>Edit2:</p>
  
  <p>System information (from <code>/etc/lsb-release</code>)</p>
  
  <pre><code>DISTRIB_ID=ManjaroLinux
  DISTRIB_RELEASE=18.0.0
  DISTRIB_CODENAME=Illyria
  DISTRIB_DESCRIPTION="Manjaro Linux"
  </code></pre>