title: trouble installing "sf" due to "gdal"
body: <p>I can't install the package "sf" on R. Seems there is an issue with gdal. No idea how to resolve it. </p>
  
  <pre><code>&gt; install.packages("sf")
  
    There is a binary version available but the source version is later:
     binary source needs_compilation
  sf  0.4-3  0.5-1              TRUE
  
  Do you want to install from sources the package which needs compilation?
  y/n: y
  installing the source package ‘sf’
  
  trying URL 'https://cran.rstudio.com/src/contrib/sf_0.5-1.tar.gz'
  Content type 'application/x-gzip' length 4073881 bytes (3.9 MB)
  ==================================================
  downloaded 3.9 MB
  
  * installing *source* package ‘sf’ ...
  ** package ‘sf’ successfully unpacked and MD5 sums checked
  configure: CC: clang
  configure: CXX: clang++
  checking for gdal-config... no
  no
  configure: error: gdal-config not found or not executable.
  ERROR: configuration failed for package ‘sf’
  * removing ‘/Library/Frameworks/R.framework/Versions/3.4/Resources/library/sf’
  Warning in install.packages :
    installation of package ‘sf’ had non-zero exit status
  
  The downloaded source packages are in
      ‘/private/var/folders/1m/w_zt5ygd26n_87h41qf574wc0000gn/T/RtmpNlYbwT/downloaded_packages’
  </code></pre>
  
  <p>Why does it say the download failed, then say the source packages are downloaded. What gives?</p>