emit a FeatureNew when using include_directories as a string
emit a FeatureNew when using include_directories as a string

This was introduced in commit 3a6e2aeed9737f1082571e868ba50e72957f27c7
as part of 0.50.0, but did not contain a FeatureNew. As a result, people
would use it without realizing that they broke support for versions of
Meson included in their minimum requirements.

mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson