Reword ELAST warning

Summary:
GCC emits a pretty amusing warning when there are apostrophes in a #warning:
```warning: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wliteral-suffix]```

Reword the warning to avoid this, and be more consistent with other warnings in libc++.

Reviewers: danalbert

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7818

llvm-svn: 230298

libcxx/src/config_elast.h

@@ -30,7 +30,7 @@
 #define _LIBCPP_ELAST _sys_nerr
 #else
 // Warn here so that the person doing the libcxx port has an easier time:
-#warning This platform's ELAST hasn't been ported yet
+#warning ELAST for this platform not yet implemented
 #endif
 
 #endif // _LIBCPP_CONFIG_ELAST