Cleanup foo.h headers and __config to work in C

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252274 91177308-0d34-0410-b5e6-96231b3b80d8

include/errno.h

@@ -31,6 +31,8 @@ Macros:
 
 #include_next <errno.h>
 
+#ifdef __cplusplus
+
 #if !defined(EOWNERDEAD) || !defined(ENOTRECOVERABLE)
 
 #ifdef ELAST
@@ -391,4 +393,6 @@ static const int __elast2 = 105;
 #define EMLINK 9979
 #endif
 
+#endif // __cplusplus
+
 #endif  // _LIBCPP_ERRNO_H