Get rid of WXCONFIG_ONLY_CPPFLAGS variable in configure

This variable was set and used exactly once, so just remove it to
simplify things.

No real changes.

configure

@@ -28671,8 +28671,6 @@ EOF
     esac
     fi
 
-            WXCONFIG_ONLY_CPPFLAGS="$WXCONFIG_ONLY_CPPFLAGS -DWXUSINGDLL"
-
     if test $wxUSE_RPATH = "no"; then
         SAMPLES_RPATH_FLAG=''
         DYLIB_PATH_POSTLINK=''
@@ -37726,8 +37724,9 @@ CXXFLAGS=`echo $WXCONFIG_CXXFLAGS $C_AND_CXX_FLAGS $CXXFLAGS `
 OBJCFLAGS=`echo $WXCONFIG_CFLAGS $CWARNINGS $C_AND_CXX_FLAGS $OBJCFLAGS `
 OBJCXXFLAGS=`echo $WXCONFIG_CXXFLAGS $C_AND_CXX_FLAGS $OBJCXXFLAGS `
 
-WXCONFIG_CPPFLAGS=`echo $WXCONFIG_CPPFLAGS $WXCONFIG_ONLY_CPPFLAGS`
-
+if test "SHARED" = 1; then
+    WXCONFIG_CPPFLAGS="$WXCONFIG_CPPFLAGS -DWXUSINGDLL"
+fi
 
 LIBS=`echo $LIBS`
 EXTRALIBS="$LDFLAGS $LDFLAGS_VERSIONING $LIBS $DMALLOC_LIBS"