change RC_BUILDIT to RC_XBS for detecting Apple build environment

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@130970 91177308-0d34-0410-b5e6-96231b3b80d8

lib/buildit

@@ -34,7 +34,7 @@ fi
 
 case $TRIPLE in
   *-apple-*)
-    if [ -z $RC_BUILDIT ]
+    if [ -z $RC_XBS ]
     then
       RC_CFLAGS="-arch i386 -arch x86_64"
     fi
@@ -79,7 +79,7 @@ case $TRIPLE in
     ;;
 esac
 
-if [ -z $RC_BUILDIT ]
+if [ -z $RC_XBS ]
 then
     rm -f libc++.1.$SOEXT*
 fi
@@ -95,7 +95,7 @@ $CC *.o $RC_CFLAGS $LDSHARED_FLAGS $EXTRA_FLAGS
 
 #libtool -static -o libc++.a *.o
 
-if [ -z $RC_BUILDIT ]
+if [ -z $RC_XBS ]
 then
     rm *.o
 fi