meson: use cross compilation compatible c++ check

src/systemd/meson.build

@@ -52,8 +52,7 @@ if cc.has_argument('-std=iso9899:2017')
         opts += [['c', '-std=iso9899:2017']]
 endif
 
-cxx = find_program('c++', required : false)
-if cxx.found()
+if add_languages('cpp', required : false)
         opts += [['c++'],
                  ['c++', '-std=c++98'],
                  ['c++', '-std=c++11']]

src/test/meson.build

@@ -957,7 +957,7 @@ tests += [
 
 ]
 
-if cxx.found()
+if cxx_cmd != ''
         tests += [
                 [['src/libsystemd/sd-bus/test-bus-vtable-cc.cc'],
                  [],