meson: create structure for use as submodule

Some other openbmc projects have set up sdbusplus to use as a
meson-subproject.  When porting one of them (entity-manager) to use
the meson build, I realized we are not structuring our meson.build
file according to best-practices for consumption as a submodule.
Specifically, we are missing the 'sdbusplus_dep' variable as described
in the meson reference: https://mesonbuild.com/Subprojects.html

Now that 'sdbusplus_dep' is defined, use it throughout the rest of
the meson.build files as well for simplification.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I79ad586aa185f769c7ecc83ef06e1f3897e8db2d
diff --git a/example/meson.build b/example/meson.build
index bd0e7c6..9b1cef4 100644
--- a/example/meson.build
+++ b/example/meson.build
@@ -1,9 +1,7 @@
 executable(
     'list-users',
     'list-users.cpp',
-    include_directories: root_inc,
-    link_with: libsdbusplus,
-    dependencies: libsystemd_pkg,
+    dependencies: sdbusplus_dep,
 )
 
 if boost_dep.found()
@@ -16,9 +14,7 @@
             '-DBOOST_ERROR_CODE_HEADER_ONLY',
             '-DBOOST_COROUTINES_NO_DEPRECATION_WARNING',
         ],
-        include_directories: root_inc,
-        link_with: libsdbusplus,
-        dependencies: [ boost_dep, pthread_dep, libsystemd_pkg ],
+        dependencies: [ boost_dep, pthread_dep, sdbusplus_dep ],
     )
 endif
 
@@ -37,7 +33,5 @@
     'calculator-server',
     'calculator-server.cpp',
     calc_files,
-    include_directories: root_inc,
-    link_with: libsdbusplus,
-    dependencies: libsystemd_pkg,
+    dependencies: sdbusplus_dep,
 )