sdbus++-gen-meson: leverage `install_dir` for custom targets

Meson has had a `install_dir` directive on `custom_target` which
allows specifying per-output where the file should be installed.
Leveraging this greatly simplifies the meson in consumers (such as
phosphor-dbus-interfaces) and makes the install less error-prone.

The previous method for installing was to use a `install_subdir`
call with large exclude lists (so that markdown did not end up in
the include tree).  The result of this was that many empty directories
were created in the include, markdown, and registry install paths.

Using this method reduces by 25% the meson content in
phosphor-dbus-interfaces.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2b03116f517caa75de902ac7e44e6923d6652cad
diff --git a/meson.build b/meson.build
index 613f9ca..8b65e92 100644
--- a/meson.build
+++ b/meson.build
@@ -21,7 +21,6 @@
   error('No valid python3 installation found')
 endif
 
-
 root_inc = include_directories('include')
 
 libsdbusplus_src = files(
@@ -72,6 +71,10 @@
     ],
 )
 
+should_generate_cpp = true
+should_generate_markdown = false
+should_generate_registry = false
+
 subdir('tools')
 
 if not get_option('examples').disabled()