Use $<INSTALL_PREFIX> in target_include_directories

To make sure packages are relocatable, use $<INSTALL_PREFIX> in
$<INSTALL_INTERFACE:...> target_include_directories.

$<INSTALL_PREFIX> was introduced in CMake 2.8.11, which is already
being checked for locally.

References:

• https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#id19
• https://github.com/robotology/how-to-export-cpp-library/blob/claudio/headeronly/src/LibTemplateCMake/CMakeLists.txt#L42

Signed-off-by: Dakota Hawkins <dakotahawkins@gmail.com>