Cleanup the C API

Author: vitaut

include/fmt/fmt-c.h

@@ -96,7 +96,8 @@ typedef signed char fmt_signed_char;
 #  else
 typedef enum {} fmt_signed_char;
 #  endif
-// Require modern MSVC with conformant preprocessor
+
+// Require modern MSVC with conformant preprocessor.
 #  if defined(_MSC_VER) && (!defined(_MSVC_TRADITIONAL) || _MSVC_TRADITIONAL)
 #    error "C API requires MSVC 2019+ with /Zc:preprocessor flag."
 #  endif
@@ -127,12 +128,12 @@ typedef enum {} fmt_signed_char;
 #  define FMT_CAT(a, b) FMT_CAT_(a, b)
 #  define FMT_CAT_(a, b) a##b
 
-#  define FMT_NARG_(_id, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, \
-                    _13, _14, _15, _16, N, ...)                             \
+#  define FMT_NARG_(_unused, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, \
+                    _12, _13, _14, _15, _16, N, ...)                       \
     N
-#  define FMT_NARG(_dummy, ...)                                                \
-    FMT_NARG_(dummy, ##__VA_ARGS__, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, \
-              4, 3, 2, 1, 0)
+#  define FMT_NARG(_unused, ...)                                             \
+    FMT_NARG_(, ##__VA_ARGS__, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, \
+              3, 2, 1, 0)
 
 #  define FMT_MAP_0(...)
 #  define FMT_MAP_1(f, a) f(a)
@@ -170,9 +171,9 @@ typedef enum {} fmt_signed_char;
 
 // select between two expressions depending on whether __VA_ARGS__ is empty
 // expands to e if __VA_ARGS__ is empty and n otherwise
-#  define FMT_VA_SELECT(e, n, ...)                                            \
-    FMT_NARG_(dummy, ##__VA_ARGS__, n, n, n, n, n, n, n, n, n, n, n, n, n, n, \
-              n, n, e)
+#  define FMT_VA_SELECT(e, n, ...)                                             \
+    FMT_NARG_(, ##__VA_ARGS__, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, \
+              e)
 
 #  define FMT_MAKE_NULL(...) NULL
 #  define FMT_MAKE_ARGLIST(...) \