Remove the cmake function:
INCLUDE_DIRECTORIES(..)
Replace it by:
TARGET_INCLUDE_DIRECTORIES(...)
It gives us the opportunity to define whether or not OpenAL dependencies
should be exported or not (using PUBLIC or PRIVATE keywoard).
[user visible changes]
The OpenAL PUBLIC headers are exported. When a target depends on OpenAL,
it will have access to its public headers.
Some small refactor along the way.
CMake 2.8.5 added the GNUInstallDirs module, which provides various
variables following the CMAKE_INSTALL_*DIR pattern to allow users a more
flexible installation setup and to provide sensible defaults while
respecting distribution specific install locations like lib64 for RPM
based linux distributions or debian multiarch tuples.
