musl only declares ecvt() under _GNU_SOURCE, so the debugger's MAD
manager fails to build against it now that gcc treats an implicit
declaration as an error. Harmless on glibc.

--- a/bld/dig/c/madman.c
+++ b/bld/dig/c/madman.c
@@ -30,6 +30,12 @@
 ****************************************************************************/
 
 
+#if defined( __LINUX__ ) && !defined( __WATCOMC__ )
+/* musl declares ecvt() only under _GNU_SOURCE, while glibc also exposes it to
+ * _XOPEN_SOURCE.  The symbol itself is present in both C libraries. */
+#define _GNU_SOURCE
+#endif
+
 #include <string.h>
 #include <ctype.h>
 #include <limits.h>