Fix make install dir error

[dreamsxin]

No description provided.

[Luke1410]

The changes related to ensure CMAKE_INSTALL_PREFIX is used for install targets is kind of a duplicate of #29 .
This change is incorporated in https://github.com/SLikeSoft/SLikeNet (GitHub and SVN repositories) and is currently scheduled to be shipped in SLikeNet 0.2.0 (though we omit the (redundant) CMAKE_INSTALL_PREFIX variable and use a slightly different relative path).

The other change, relating to adding the -fPIC compiler parameter looks correct, but would be an ABI breaking change. Therefore we are considering incorporating the change in the 2.x line, but not in the 0.x/1.x line atm.

[dreamsxin]

Thank you.