title: cmake ignore exact option in findPackage for protobuf
body: <p>I have a problem with cmake</p>
  
  <p>I'm writting in CMakeLists <br /></p>
  
  <p>set(PROTOBUF_VERSION "2.4.1") <br />
  find_package(Protobuf ${PROTOBUF_VERSION} EXACT REQUIRED)</p>
  
  <p>But when I run cmake on my machine with protobuf 2.5.0, it successfully generates makefile. 
  <br />In stdout I have only:<br /></p>
  
  <p><code>
  -- Found PROTOBUF: /usr/local/lib/libprotobuf.so
  </code></p>
  
  <p>But for ZLIB I have <br /></p>
  
  <p><code>
  -- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.7")
  </code></p>
  
  <p>Probably, protobuf doesn't contain a version of itself in the library.
  Is there a way to specify protobufer's version?</p>