use unsetenv() instead of setenv(var, NULL) which is invalid

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@47942 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

setup.h_vms

@@ -897,6 +897,7 @@ typedef pid_t GPid;
 /* Define if you have the setenv function.  */
 #   if __CRTL_VER >= 70000000
 #define HAVE_SETENV 1
+#define HAVE_UNSETENV 1
 #else
 #undef HAVE_SETENV
 #endif