feat(build): install/export rules, umbrella header, verified consumer

adapters/gtk4/CMakeLists.txt

@@ -22,13 +22,33 @@ endif()
 
 add_library(plot_gtk4 STATIC gtk_plot_area.cpp gtk_plot_area.hpp gtk_input.hpp)
 add_library(plot::gtk4 ALIAS plot_gtk4)
+# The installed tree names it plot::gtk4, like the build tree (see plot_core's).
+set_target_properties(plot_gtk4 PROPERTIES EXPORT_NAME gtk4)
+
 # The public include root is `adapters/`, so the headers are reached as
 # <gtk4/gtk_plot_area.hpp> and <gtk4/gtk_input.hpp> -- the toolkit's name first,
-# like every other adapter's.
-target_include_directories(plot_gtk4 PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# like every other adapter's. The build interface is that root; the install one is
+# the prefix's `include`, where both are installed under `gtk4/`, the same
+# spelling.
+target_include_directories(plot_gtk4 PUBLIC
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  $<INSTALL_INTERFACE:include>)
 # GTK is PUBLIC, unlike GLFW and ImGui: the adapter's factory hands out a
 # GtkWidget, so a consumer that embeds the area needs GTK's headers and libraries
 # to put it anywhere -- the widget *is* the toolkit's object -- and GTK's include
 # directory is not one the compiler searches by default.
 target_link_libraries(plot_gtk4 PUBLIC plot::core PkgConfig::GTK4_PC)
 plot_apply_warnings(plot_gtk4)
+
+# GTK came from pkg-config, which the installed config file can re-run: the
+# snippet re-creates the exact PkgConfig::GTK4_PC target the adapter's public link
+# interface names, so a consumer that links plot::gtk4 gets GTK's own include
+# directories and libraries the same way this build did.
+plot_record_adapter_export(plot_gtk4 TRUE "GTK4 was resolved by pkg-config"
+  "find_dependency(PkgConfig)\npkg_check_modules(GTK4_PC REQUIRED IMPORTED_TARGET gtk4)\n")
+if(PLOT_INSTALL)
+  # gtk_input.hpp is installed with the public header: the demo and the test suite
+  # reach it as <gtk4/gtk_input.hpp>, and it is a toolkit-free header a consumer
+  # can use without GTK.
+  install(FILES gtk_plot_area.hpp gtk_input.hpp DESTINATION include/gtk4)
+endif()