Fix install interface to use CMAKE_INSTALL_INCLUDEDIR

Currently, the sparta target will use `$CMAKE_INSTALL_PREFIX/include` as
the include directory when installed. If the user sets a custom
CMAKE_INSTALL_INCLUDEDIR, however, then this will not be the correct
location. This commit changes the include directory of the sparta target
to be consistent with the actual installed location.

CMakeLists.txt

@@ -26,7 +26,7 @@ add_library(sparta INTERFACE)
 
 target_include_directories(sparta INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-  $<INSTALL_INTERFACE:include>
+  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
 target_link_libraries(sparta INTERFACE ${Boost_LIBRARIES})