title: CMake CMAKE_INSTALL_PREFIX completely ignored
body: <p>I'm trying to build protobuf on my windows machine according to <a href="https://github.com/protocolbuffers/protobuf/blob/master/cmake/README.md" rel="nofollow noreferrer">this guide</a> (official proto guide). Compiling works fine. But. I would like to install it in my workspace, where other project can have easy access to it. According to the guide I should specify the install prefix like so:</p>
  <pre><code> C:\Path\to\protobuf\cmake\build\release&gt;cmake -G &quot;NMake Makefiles&quot; ^
   -DCMAKE_BUILD_TYPE=Release ^
   -DCMAKE_INSTALL_PREFIX=../../../../install ^
   ../..
  </code></pre>
  <p>This does not have the desired effect as the end result installs in C:\ .
  Also if I use an env variable:</p>
  <pre><code> $Env:CMAKE_INSTALL_PREFIX=&quot;C:/Users/uname/Workspace/protobuf-3.18.0/install&quot;
   cmake -G &quot;NMake Makefiles&quot; -DCMAKE_BUILD_TYPE=Release ..\..
  </code></pre>
  <p>Or</p>
  <pre><code> $Env:CMAKE_INSTALL_PREFIX=&quot;C:\Users\uname\Workspace\protobuf-3.18.0\install&quot;
   cmake -G &quot;NMake Makefiles&quot; -DCMAKE_BUILD_TYPE=Release ..\..
  </code></pre>
  <p>I get a slightly different outcome. It tries to install in:</p>
  <pre><code>C:\Program files (x86)\Protobuf\lib
  </code></pre>
  <p>And off course fails due to lack of priveliges. Is it possible to arrange the env in powershell in such a way that cmake actually picks up the install prefix <em><strong>that <em><strong>I</strong></em> want</strong></em>?</p>