https://bugs.gentoo.org/981271

The header inclusion on its own is upstreamable, but the rest are getting into the unsupported territory.
Ideally musl locales would be properly supported, I looked at the options and build vise it can be done.
I stopped with that attempt as locales refused to work at runtime for Musl.

Maybe when getlocalename_l gets buy-in from Glibc and Musl this could be revisited.
https://austingroupbugs.net/view.php?id=1220
https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/functions/getlocalename_l.html

See also
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/community/mlt/musl.patch

--- a/src/melt/io.c
+++ b/src/melt/io.c
@@ -27,6 +27,7 @@
 #include <stdlib.h>
 #include <string.h>
 #ifndef _WIN32
+#include <sys/time.h>
 #include <termios.h>
 #include <unistd.h>
 #else
--- a/src/tests/test_filter/test_filter.cpp
+++ b/src/tests/test_filter/test_filter.cpp
@@ -28,7 +28,7 @@ class TestFilter : public QObject
 public:
     TestFilter()
     {
-#if defined(__linux__) || defined(__APPLE__)
+#if defined(__GLIBC__) || defined(__APPLE__)
         locale = newlocale(LC_NUMERIC_MASK, "POSIX", NULL);
 #endif
         Factory::init();
@@ -36,7 +36,7 @@ public:
 
     ~TestFilter()
     {
-#if defined(__linux__) || defined(__APPLE__)
+#if defined(__GLIBC__) || defined(__APPLE__)
         freelocale(locale);
 #endif
     }
--- a/src/tests/test_properties/test_properties.cpp
+++ b/src/tests/test_properties/test_properties.cpp
@@ -24,7 +24,6 @@
 using namespace Mlt;
 
 extern "C" {
-#define __APPLE__
 #include <framework/mlt_animation.h>
 #include <framework/mlt_property.h>
 }
@@ -547,6 +546,7 @@ private Q_SLOTS:
 
     void RadixRespondsToLocale()
     {
+#if !defined(_WIN32) && (defined(__GLIBC__) || defined(__APPLE__))
         Properties p;
         p.set_lcnumeric("en_US.UTF-8");
         p.set("key", "0.125");
@@ -555,6 +555,9 @@ private Q_SLOTS:
         p.set_lcnumeric("de_DE.UTF-8");
         p.set("key", "0,125");
         QCOMPARE(p.get_double("key"), double(1) / double(8));
+#endif
+#else
+        QSKIP("Doesn't work on Musl");
 #endif
     }