Fix meson warnings

As written, when we pull in the boost and boost-url subprojects, we hit
problems in the meson files.

```
../meson.build:291: WARNING: include_directories sandbox violation!
```
This commit resolves this issue, by adding explicit meson.build files
for both boost and boost-url.

Tested:
meson buildlocal

No longer returns the above error, and shows
  Subprojects
    boost                                 : YES
    boost-url                             : YES

Whereas previously those two dependencies showed up as NO.

Code compiles.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ib25340723c8cb7d6139e3e51db023e9d90e30aab
diff --git a/meson.build b/meson.build
index 8c86fa9..12e51a3 100644
--- a/meson.build
+++ b/meson.build
@@ -287,20 +287,14 @@
 
 boost = dependency('boost',version : '>=1.79.0', required : false, include_type: 'system')
 if not boost.found()
-  subproject('boost', required: false)
-  boost_inc = include_directories('subprojects/boost_1_80_0/', is_system:true)
-  boost  = declare_dependency(include_directories : boost_inc)
-  boost = boost.as_system('system')
+  boost = subproject('boost', required: true).get_variable('boost_dep')
 endif
 bmcweb_dependencies += boost
 
 if cxx.has_header('boost/url/url_view.hpp')
   boost_url = declare_dependency()
 else
-  subproject('boost-url', required: false)
-  boost_url_inc = include_directories('subprojects/boost-url/include', is_system:true)
-  boost_url= declare_dependency(include_directories : boost_url_inc)
-  boost_url = boost_url.as_system('system')
+  boost_url = subproject('boost-url', required: true).get_variable('boost_url_dep')
 endif
 bmcweb_dependencies += boost_url