Add CONFIG_FREQUENCY_STD_FORMAT flag

Author: cleishm

Kconfig

@@ -0,0 +1,11 @@
+menu "Frequency Library"
+
+    config FREQUENCY_STD_FORMAT
+        bool "Enable std::format support"
+        default y
+        help
+            Include <format> and provide std::formatter specializations
+            for frequency types. Disable if <format> is unavailable or
+            undesired.
+
+endmenu

include/frequency/frequency.hpp

@@ -12,7 +12,15 @@
 #include <limits>
 #include <ratio>
 #include <string>
+#ifndef CONFIG_FREQUENCY_STD_FORMAT
 #if __has_include(<format>) && defined(__cpp_lib_format)
+#define CONFIG_FREQUENCY_STD_FORMAT 1
+#else
+#define CONFIG_FREQUENCY_STD_FORMAT 0
+#endif
+#endif
+
+#if CONFIG_FREQUENCY_STD_FORMAT
 #include <format>
 #endif
 #include <assert.h>
@@ -1130,7 +1138,7 @@ struct common_type<freq::frequency<Rep1, Precision1>, freq::frequency<Rep2, Prec
     using type = freq::frequency<std::common_type_t<Rep1, Rep2>, common_precision>;
 };
 
-#if __has_include(<format>) && defined(__cpp_lib_format)
+#if CONFIG_FREQUENCY_STD_FORMAT
 template<>
 struct formatter<freq::millihertz> {
     constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }