sdbus++-gen-meson: Don't build markdown by default

Not all builds want the markdown (notably subproject builds) and we can
trivially create a target to build and install it.

```
custom_target(
    'md',
    command: 'true',
    output: 'md',
    capture: true,
    depends: generated_others,
    build_by_default: true)
```

This increments the version counter to ensure users notice the behavior
change.

Change-Id: I55fd995d8ab27b2f06d3c382b3379bd36ba10c6f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/tools/sdbus++-gen-meson b/tools/sdbus++-gen-meson
index cb48187..e2da6eb 100755
--- a/tools/sdbus++-gen-meson
+++ b/tools/sdbus++-gen-meson
@@ -36,7 +36,7 @@
 ## if a repository contains old copies of the generated meson.build files and
 ## needs an update.  We should increment the version number whenever the
 ## resulting meson.build would change.
-tool_version="sdbus++-gen-meson version 2"
+tool_version="sdbus++-gen-meson version 3"
 function show_version {
     echo "$tool_version"
 }
@@ -239,7 +239,6 @@
         '--directory', meson.current_source_dir() / '$yamldir',
         '$1',
     ],
-    build_by_default: true,
 )
 
 EOF