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 **
默認情況下，包會被安裝在/usr/local路徑下。但是，在某些系統平臺上(如Ubuntu 11)，/usr/local/lib路徑并不包含在LD_LIBRARY_PATH中。你可以手動添加，但也可以很方便的通過安裝到/usr路徑下，調 用configure命令參數如下
./configure --prefix=/usr
如果你的包已經編譯過，確保先運行"make clean"再重新編譯。
reference:
http://code.google.com/p/protobuf/issues/detail?id=213