Fix CMake files on centos 6

On Centos 6, the CMake files is broken. I have a fix and have tested on Centos 6.5.
@ozancaglayan I mean: On Centos 6, if you install "lmdb-devel" library using "yum install lmdb-devel". The installed library is too old. The compilation failed. You need to manually install "lmdb" library from source code as follows:

git clone git://gitorious.org/mdb/mdb.git
cd mdb/libraries/liblmdb
make && make install