Add a patch from the Alpine Linux project to fix a musl build issue with gcc 5: 

Problem has been reported upstream and closed with WONTFIX:
http://bugs.musicpd.org/view.php?id=4387
http://bugs.musicpd.org/view.php?id=4110

however...

POSIX does not permit using PTHREAD_COND_INITIALIZER except for static
initialization, and certainly does not permit using it as a value

POSIX does not specify the type of the object (it's opaque) so if
there are any types for which their code would be invalid C++, then their
code is invalid

Volatile in the type is necessary. without that, LTO can break the code.

--- a/src/notify.hxx
+++ b/src/notify.hxx
@@ -28,7 +28,7 @@ struct notify {
 	Cond cond;
 	bool pending;
 
-#if !defined(WIN32) && !defined(__NetBSD__) && !defined(__BIONIC__)
+#if defined(__GLIBC__)
 	constexpr
 #endif
 	notify():pending(false) {}
--- a/src/thread/PosixCond.hxx
+++ b/src/thread/PosixCond.hxx
@@ -41,7 +41,7 @@ class PosixCond {
 	pthread_cond_t cond;
 
 public:
-#if defined(__NetBSD__) || defined(__BIONIC__)
+#if !defined(__GLIBC__)
 	/* NetBSD's PTHREAD_COND_INITIALIZER is not compatible with
 	   "constexpr" */
 	PosixCond() {
--- a/src/thread/PosixMutex.hxx
+++ b/src/thread/PosixMutex.hxx
@@ -41,7 +41,7 @@ class PosixMutex {
 	pthread_mutex_t mutex;
 
 public:
-#if defined(__NetBSD__) || defined(__BIONIC__)
+#if !defined(__GLIBC__)
 	/* NetBSD's PTHREAD_MUTEX_INITIALIZER is not compatible with
 	   "constexpr" */
 	PosixMutex() {