Install gio-launch-desktop in a non-PATH location

This is an internal helper executable, which users shouldn't invoke
directly (see glib#1633).

When building for a single-architecture distribution, we can install
it as ${libexecdir}/gio-launch-desktop.

When building for a multiarch distribution, installing it into an
architecture-specific location and packaging it alongside the GLib
library avoids the problem discussed in glib#1633 where it would either
cause a circular dependency between the GLib library and a common
cross-architecture package (libglib2.0-bin in Debian), or require a
separate package just to contain gio-launch-desktop, or cause different
architectures' copies to overwrite each other.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/gdesktopappinfo.c

@@ -2926,6 +2926,10 @@ g_desktop_app_info_launch_uris_with_spawn (GDesktopAppInfo            *info,
           if (!is_setuid)
             tmp = g_getenv ("GIO_LAUNCH_DESKTOP");
 
+          /* Allow build system to specify path to gio-launch-desktop */
+          if (tmp == NULL && g_file_test (GIO_LAUNCH_DESKTOP, G_FILE_TEST_IS_EXECUTABLE))
+            tmp = GIO_LAUNCH_DESKTOP;
+
           /* Fall back on usual searching in $PATH */
           if (tmp == NULL)
             tmp = "gio-launch-desktop";