libs/rem: fix build with musl

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

libs/rem/patches/100-no-pthread_rwlockattr_setkind_np-on-musl.patch

@@ -0,0 +1,13 @@
+Index: rem-0.4.6/src/vidmix/vidmix.c
+===================================================================
+--- rem-0.4.6.orig/src/vidmix/vidmix.c
++++ rem-0.4.6/src/vidmix/vidmix.c
+@@ -343,7 +343,7 @@ int vidmix_alloc(struct vidmix **mixp)
+ 		return err;
+ 	}
+ 
+-#ifdef LINUX
++#if defined(LINUX) && defined(__GLIBC__)
+ 	err = pthread_rwlockattr_setkind_np(&attr,
+ 				 PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
+ 	if (err)