[c]: install headers under cucumber/gherkin/ and .cmake files under gherkin/ (#346)

* [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).

* [c]: install cmake configs under gherkin/

Rather than installing `.cmake` files directly under e.g.
`/usr/lib/cmake`, put them in a subdirectory like
`/usr/lib/cmake/gherkin`.

C++ CMake files are currently installed under e.g.
`/usr/lib/cmake/cucumber_gherkin`; I think the leading `cucumber_`
corresponds to the `cucumber::` C++ namespace, which is why I have
chosen just `gherkin` for the C CMake files.

Author: musicinmybrain

c/CMakeLists.txt

@@ -155,14 +155,14 @@ install(
 
 install(
   DIRECTORY "${PROJECT_SOURCE_DIR}/include/"
-  DESTINATION ${include_install_dir}
+  DESTINATION ${include_install_dir}/cucumber/gherkin
   FILES_MATCHING PATTERN "*.*h")
 
 install(
     FILES "${project_config}"
-    DESTINATION "${config_install_dir}")
+    DESTINATION "${config_install_dir}/gherkin")
 
 install(
     EXPORT "${targets_export_name}"
     NAMESPACE "${namespace}"
-    DESTINATION "${config_install_dir}")
+    DESTINATION "${config_install_dir}/gherkin")