Remove unnecessary DLLs from Windows builds

This saves 21 MiB of MSYS2 libraries, without any adverse effects.

The MSYS2 build remains bloated, due to the Adwaita icon theme.

cmake/Win32Cleanup.cmake

@@ -0,0 +1,37 @@
+# To be run from cmake_install.cmake, eradicates all unreferenced libraries.
+# CMake 3.9.6 has a parsing bug with ENCODING UTF-8.
+cmake_minimum_required (VERSION 3.10)
+
+# CPack runs this almost without any CMake variables at all
+# (cmStateSnapshot::SetDefaultDefinitions(), CMAKE_INSTALL_PREFIX, [DESTDIR])
+set (installdir "${CMAKE_INSTALL_PREFIX}")
+if (NOT installdir OR installdir MATCHES "^/usr(/|$)")
+	return ()
+endif ()
+
+# The function is recursive and CMake has tragic scoping behaviour;
+# environment variables are truly global there, in the absence of a cache
+unset (ENV{seen})
+function (expand path)
+	set (seen $ENV{seen})
+	if (path IN_LIST seen OR NOT EXISTS "${path}")
+		return ()
+	endif ()
+
+	set (ENV{seen} "$ENV{seen};${path}")
+	file (STRINGS "${path}" strings REGEX "[.][Dd][Ll][Ll]$" ENCODING UTF-8)
+	foreach (string ${strings})
+		string (REGEX MATCH "[-.+_a-zA-Z0-9]+$" word "${string}")
+		expand ("${installdir}/${word}")
+	endforeach ()
+endfunction ()
+
+file (GLOB roots LIST_DIRECTORIES false "${installdir}/*.[Ee][Xx][Ee]"
+	"${installdir}/lib/gdk-pixbuf-2.0/2.10.0/loaders/*.[Dd][Ll][Ll]")
+foreach (binary ${roots})
+	expand ("${binary}")
+endforeach ()
+
+file (GLOB libraries LIST_DIRECTORIES false "${installdir}/*.[Dd][Ll][Ll]")
+list (REMOVE_ITEM libraries $ENV{seen})
+file (REMOVE ${libraries})