title: PKG_CONFIG_PATH environment variable (install ipopt error)
body: <p>I tried to install <code>ipopt</code> trough <code>pip install ipopt</code></p>
  
  <p>Here is my error:</p>
  
  <pre><code>    Collecting ipopt
    Using cached ipopt-0.2.0.tar.gz (162 kB)
      ERROR: Command errored out with exit status 1:
       command: /usr/local/opt/python/bin/python3.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-install-i9o4pqdg/ipopt/setup.py'"'"'; __file__='"'"'/private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-install-i9o4pqdg/ipopt/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-pip-egg-info-_gej3ysb
           cwd: /private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-install-i9o4pqdg/ipopt/
      Complete output (11 lines):
      Package ipopt was not found in the pkg-config search path.
      Perhaps you should add the directory containing `ipopt.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'ipopt' found
      Traceback (most recent call last):
        File "&lt;string&gt;", line 1, in &lt;module&gt;
        File "/private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-install-i9o4pqdg/ipopt/setup.py", line 102, in &lt;module&gt;
          **pkgconfig('ipopt'))]
        File "/private/var/folders/3w/zf3x85h55tv7qk2yg8ykztwc0000gn/T/pip-install-i9o4pqdg/ipopt/setup.py", line 58, in pkgconfig
          raise OSError(msg.format(list(packages)))
      OSError: pkg-config was not able to find any of the requested packages ['ipopt'] on your system. Make sure pkg-config can discover the .pc files associated with the installed packages.
      ----------------------------------------
  ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  </code></pre>
  
  <p>Command <code>pkg-config --variable pc_path pkg-config</code> gives:</p>
  
  <pre><code>/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/local/Homebrew/Library/Homebrew/os/mac/pkgconfig/10.15
  </code></pre>
  
  <p>Made this (but I think that the link is strange), but didn't solve the problem</p>
  
  <pre><code>export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/local/Homebrew/Library/Homebrew/os/mac/pkgconfig/10.15
  </code></pre>