build(libconstants): changed over to use external libconstants

this is the same module as before just broken out into its own subproject

build-config/fourdst/libconstants/meson.build

@@ -0,0 +1,2 @@
+const_p = subproject('libconstants')
+const_dep  = const_p.get_variable('const_dep')

build-config/fourdst/meson.build

@@ -0,0 +1 @@
+subdir('libconstants')

build-config/meson.build

@@ -1,5 +1,7 @@
 cmake = import('cmake')
 
+subdir('fourdst')
+
 subdir('mfem')
 subdir('yaml-cpp')
 subdir('quill')
@@ -20,4 +22,4 @@ elif configErr == 'harsh'
     commonCppArgs += ['-DCONFIG_HARSH']
 endif
 
-add_project_arguments(commonCppArgs, language: 'cpp')
+add_project_arguments(commonCppArgs, language: 'cpp')