[libc] Fix missing default value for errno config (#100175)

Summary:
The configs all need default values which targets then override. This
one was an empty string which made the logic report an error. The only
reason it wasn't a build failure was because of a stray `:`.

Author: jhuber6

libc/cmake/modules/LibcConfig.cmake

@@ -113,7 +113,7 @@ function(load_libc_config config_file)
       message(FATAL_ERROR ${json_error})
     endif()
     if(NOT DEFINED ${opt_name})
-      message(FATAL_ERROR: " Option ${opt_name} defined in ${config_file} is invalid.")
+      message(FATAL_ERROR " Option ${opt_name} defined in ${config_file} is invalid.")
     endif()
     if(ARGN)
       list(FIND ARGN ${opt_name} optname_exists)

libc/config/config.json

@@ -1,8 +1,8 @@
 {
   "errno": {
     "LIBC_CONF_ERRNO_MODE": {
-      "value": "",
-      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM."
+      "value": "LIBC_ERRNO_MODE_DEFAULT",
+      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM."
     }
   },
   "printf": {

libc/src/errno/libc_errno.cpp

@@ -9,6 +9,8 @@
 #include "libc_errno.h"
 #include "src/__support/macros/config.h"
 
+// libc uses a fallback default value, either system or thread local.
+#define LIBC_ERRNO_MODE_DEFAULT 0
 // libc never stores a value; `errno` macro uses get link-time failure.
 #define LIBC_ERRNO_MODE_UNDEFINED 1
 // libc maintains per-thread state (requires C++ `thread_local` support).
@@ -23,20 +25,23 @@
 // fullbuild mode, effectively the same as `LIBC_ERRNO_MODE_EXTERNAL`.
 #define LIBC_ERRNO_MODE_SYSTEM 5
 
-#ifndef LIBC_ERRNO_MODE
+#if !defined(LIBC_ERRNO_MODE) || LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_DEFAULT
+#undef LIBC_ERRNO_MODE
 #if defined(LIBC_FULL_BUILD) || !defined(LIBC_COPT_PUBLIC_PACKAGING)
 #define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_THREAD_LOCAL
 #else
 #define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
 #endif
 #endif // LIBC_ERRNO_MODE
 
-#if LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_UNDEFINED &&                            \
+#if LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_DEFAULT &&                              \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_UNDEFINED &&                            \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_THREAD_LOCAL &&                         \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SHARED &&                               \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_EXTERNAL &&                             \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM
 #error LIBC_ERRNO_MODE must be one of the following values: \
+LIBC_ERRNO_MODE_DEFAULT, \
 LIBC_ERRNO_MODE_UNDEFINED, \
 LIBC_ERRNO_MODE_THREAD_LOCAL, \
 LIBC_ERRNO_MODE_SHARED, \