[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.

c/CMakeLists.txt

@@ -159,9 +159,9 @@ install(
 
 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")