build system: Add MACRO_DEPRECATED macro

Adding this macro in the definition of a macro causes a warning about
the deprecation to be emitted when used (and a build failure with
`WERROR=1`). This is useful as no other means to deprecate preprocessor
macros are provided.

The macro will be defined empty for compilers that are not GCC or
clang.

dist/tools/genconfigheader/riotbuild-prefix.h.in

@@ -62,4 +62,23 @@
  */
 #define CPU_RAM_SIZE        /* RAM Size in Bytes */
 #endif /* DOXYGEN */
+
+/**
+ * @brief   Mark a preprocessor macro as deprecated by including this
+ *          macro in the definition
+ *
+ * Usage:
+ * ``` C
+ * /// @deprecated, use @ref BAR instead of FOO
+ * #define FOO          MACRO_DEPRECATED BAR
+ * ```
+ */
+#if defined(DOXYGEN)
+#  define MACRO_DEPRECATED    /* implementation */
+#elif defined(__GNUC__) || defined(__clang__)
+#  define MACRO_DEPRECATED _Pragma("GCC warning \"Code is using a deprecated macro\"")
+#else
+#  define MACRO_DEPRECATED
+#endif
+
 /** @} */