Use cd -P

Setting PWD in the process does not let `cd` traverse symlinks in
/bin/sh on macOS.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64906 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -262,7 +262,7 @@ AS_CASE(["$build_os"],
 	@%:@error ignore linker warnings
 	@%:@endif
 SRC
-	CC_WRAPPER=`PWD=; cd "$srcdir/tool" && pwd`/darwin-cc
+	CC_WRAPPER=`cd -P "$srcdir/tool" && pwd`/darwin-cc
 	CC="$CC_WRAPPER $CC"
     ])
   ])