build: Add option to disable libphosphor_dbus

Add an option to disable libphosphor_dbus (YAML only).

Change-Id: I94107e4fede2668c29ce101d339b0488e92cefca
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/Makefile.am b/Makefile.am
index f00e7d1..464c668 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,5 +1,6 @@
 AM_DEFAULT_SOURCE_EXT = .cpp
 
+if WANT_LIBPHOSPHOR_DBUS
 nobase_include_HEADERS = ## These get filled in by Makefile.interfaces
 
 libphosphor_dbusdir = ${libdir}
@@ -16,6 +17,7 @@
 clean-local: clean-dbus
 
 -include Makefile.interfaces
+endif
 
 yamldir = ${pkgdatadir}/yaml
 nobase_yaml_DATA = ## Filled in by Makefile.yaml