title: CMake with gRPC cannot find gRPCTargets.cmake
body: <p>I am having issues building the <code>grpc</code> <code>cpp</code> <code>helloworld</code> example with <code>cmake</code>.
  I built and installed <code>grpc</code> with <code>cmake</code> initially, and then with <code>make</code> directly.</p>
  
  <p>I have found <a href="https://github.com/grpc/grpc/issues/11378" rel="noreferrer">this</a> issue raised by someone else in the past, which was closed as resolved.<br>
  It does not appear to be resolved and I opened a new <a href="https://github.com/grpc/grpc/issues/19873" rel="noreferrer">issue</a> for it, but I feel it will be some time until I get some help, so here I am.</p>
  
  <p>The OP of the original issue offers a workaround with his <a href="https://github.com/zhengwx11/FindGRPC" rel="noreferrer">FindGRPC</a> cmake module, but I am not sure how is this suppose to help if <code>gRPCTargets.cmake</code> is still missing.<br>
  I dropped <code>FindGRPC.cmake</code> inside my cmake modules path, but nothing changes.</p>
  
  <p>The error is this:</p>
  
  <pre><code>CMake Error at /usr/local/lib/cmake/grpc/gRPCConfig.cmake:8 (include):
    include could not find load file:
  
      /usr/local/lib/cmake/grpc/gRPCTargets.cmake
  Call Stack (most recent call first):
    CMakeLists.txt:73 (find_package)
  
  
  -- Using gRPC 1.20.0
  -- Configuring incomplete, errors occurred
  </code></pre>
  
  <p>I want to be able to use <code>grpc</code> from my cmake projects without much hassle (using <code>find_package(gRPC CONFIG REQUIRED)</code>)</p>
  
  <p><em>EDIT:</em></p>
  
  <p>When running <code>cmake</code> on <code>grpc</code> I get this error:</p>
  
  <pre><code>gRPC_INSTALL will be forced to FALSE because gRPC_ZLIB_PROVIDER is "module"
  </code></pre>
  
  <p>This is printed from <code>zlib.cmake</code>:</p>
  
  <pre><code>message(WARNING "gRPC_INSTALL will be forced to FALSE because gRPC_ZLIB_PROVIDER is \"module\"")
  </code></pre>
  
  <p>Apparently all the providers must be <code>"package"</code> as mentioned in <code>grpc</code>'s <code>CMakeLists.txt</code>:</p>
  
  <pre><code>set(gRPC_INSTALL ${gRPC_INSTALL_default} CACHE BOOL
      "Generate installation target: gRPC_ZLIB_PROVIDER, gRPC_CARES_PROVIDER, gRPC_SSL_PROVIDER and gRPC_PROTOBUF_PROVIDER must all be \"package\"")
  </code></pre>
  
  <p>I am not sure why <code>zlib</code> is a module here though, or how make it a package.<br>
  Do I need to somehow to specify to <code>cmake</code> to use the installed zlib instead of the submodule one?</p>