rebased cmake updates (#147)

rebased cmake updates (#147)

* Use a target property instead of CMAKE_C_FLAGS

It is a global property, which is not great when importing utf8proc in other CMake projects.

* Use target properties instead of add_definitions

It modifies builds settings globally, which is not great when using
the library as part of a bigger CMake build

* Expose header search path with target_include_directories

This makes it possible to use utf8proc in a larger CMake based project
with

add_subdirectory(utf8proc)
target_link_libraries(my_app utf8proc)

* more rebase fixes