Fix build when linking against musl

musl doesn't define this macro, so it has to be done manually.
Since there doesn't seem to be a macro to detect whether musl is used,
we should always define the macro on Linux.

Author: Calinou

src/VHACD_Lib/inc/vhacdMutex.h

@@ -71,7 +71,7 @@
 #include <pthread.h>
 #endif
 
-#if defined(__APPLE__)
+#if defined(__APPLE__) || defined(__linux__)
 #define PTHREAD_MUTEX_RECURSIVE_NP PTHREAD_MUTEX_RECURSIVE
 #endif