site.def: add define guards around DtLocalesToBuild

cde/config/cf/site.def

@@ -113,13 +113,17 @@ XCOMM build the DtXinerama support
 #define CDE_USEXINERAMA         YES
 
 #ifdef SunArchitecture
-# define DtLocalesToBuild de_DE.UTF-8 es_ES.UTF-8 fr_FR.UTF-8 it_IT.UTF-8
+# if !defined(DtLocalesToBuild)
+#  define DtLocalesToBuild en_US.UTF-8 de_DE.UTF-8 es_ES.UTF-8 fr_FR.UTF-8 it_IT.UTF-8
+# endif
 #endif
 
 XCOMM Make sure you have installed the de_DE, es_ES, fr_FR and it_IT locales.
 XCOMM  or building non-C locales will cause build failures.
 #if defined(BSDArchitecture)
-# define DtLocalesToBuild de_DE.UTF-8 es_ES.UTF-8 fr_FR.UTF-8 it_IT.UTF-8
+# if !defined(DtLocalesToBuild)
+#  define DtLocalesToBuild en_US.UTF-8 de_DE.UTF-8 es_ES.UTF-8 fr_FR.UTF-8 it_IT.UTF-8
+# endif
 #endif
 
 #if defined(LinuxArchitecture)