Please follow the instructions in README.txt:

** Hint on install location **

  By default, the package will be installed to /usr/local.  However,
  on many platforms, /usr/local/lib is not part of LD_LIBRARY_PATH.
  You can add it, but it may be easier to just install to /usr
  instead.  To do this, invoke configure as follows:

    ./configure --prefix=/usr

  If you already built the package with a different prefix, make sure
  to run "make clean" before building again.

** 翻譯 translate **

  默認(rèn)情況下，包會(huì)被安裝在/usr/local路徑下。但是，在某些系統(tǒng)平臺(tái)上(如Ubuntu 11)，/usr/local/lib路徑并不包含在LD_LIBRARY_PATH中。你可以手動(dòng)添加，但也可以很方便的通過(guò)安裝到/usr路徑下，調(diào) 用configure命令參數(shù)如下

    ./configure --prefix=/usr

  如果你的包已經(jīng)編譯過(guò)，確保先運(yùn)行"make clean"再重新編譯。

reference:http://code.google.com/p/protobuf/issues/detail?id=213