[c]: install headers under cucumber/gherkin/

Rather than installing all the C headers directly under e.g.
`/usr/include`, put them in a subdirectory like
`/usr/include/cucumber/gherkin`.

This is *not* the same directory as the C++ headers, which have an extra
`cucumber/` corresponding to the `cucumber::` C++ namespace, e.g.
`/usr/include/cucumber/cucumber/gherkin/`. See
cucumber/messages#267 (comment).

Author: musicinmybrain

c/CMakeLists.txt

@@ -154,7 +154,7 @@ install(
 
 install(
   DIRECTORY "${PROJECT_SOURCE_DIR}/include/"
-  DESTINATION ${include_install_dir}
+  DESTINATION ${include_install_dir}/cucumber/gherkin
   FILES_MATCHING PATTERN "*.*h")
 
 install(