Allows the user of owl to exclude the samples.

With 3f805af you forced cmake to add the samples as targets even if OWL_BUILD_SAMPLES is OFF . We're using Visual Studio and our cmake view gets cluttered with all those targets.

With this change the samples are not added, when owl is a subproject. If one would like to get the samples OWL_BUILD_SAMPLES must be set beforehand.

Examples tested with Windows and VS.

We don't want the samples but the owl viewer:

find_package(glfw3 CONFIG REQUIRED)
set(owl_dir ${CMAKE_CURRENT_LIST_DIR}/owl)
add_subdirectory(${owl_dir} EXCLUDE_FROM_ALL)
add_subdirectory(${owl_dir}/samples/common)

If one would like to add the samples when owl is a subproject:

set(owl_dir ${CMAKE_CURRENT_LIST_DIR}/owl)
option(OWL_BUILD_SAMPLES ON)
add_subdirectory(${owl_dir} EXCLUDE_FROM_ALL)