Using CMAKE_INSTALL_PREFIX in install command is redundant.

From documentation (http://www.cmake.org/cmake/help/v2.8.12/cmake.html#command:install):
    If a relative path is given it is interpreted relative to the value of CMAKE_INSTALL_PREFIX.
Merge pull request #44 from ruslo/install.fix

Using CMAKE_INSTALL_PREFIX in install command is redundant.
Merge remote-tracking branch 'upstream/master'