Mark target compile options/features as private

[nega0]

Marks target_compile_options and target_compiler features as PRIVATE. If
marked as PUBLIC, then in photosplineConfig.cmake,
set_target_properties(photospline ...) INTERFACE_COMPILE_OPTIONS and
INTERFACE_COMPLE_FEATURES are set. This tells consumers of photospline
(that use CMake) to also use these options/features.

[nega0]

aw geez, word-wrap got hosed on that commit. i can fix that prior to pulling

[cnweaver]

This will work, but it will also mean that users will need to take on the responsibility of figuring out all of the CPU feature/vectorization flags they need to get good performance (bearing in mind that many of the critical hot portions of the code are in header files, so they optimized when the code using photospline is compiled, not just when photospline itself is compiled).