main/mpd: document the musl gcc fix

underlying problem has been reported upstream:
http://bugs.musicpd.org/view.php?id=4387
http://bugs.musicpd.org/view.php?id=4110

main/mpd/APKBUILD

@@ -77,16 +77,16 @@ package() {
 }
 md5sums="da4bc3e47afd0faf9e7a67168e012102  mpd-0.19.10.tar.xz
 a84cce5fdb848c7d282e38698432df51  underlinking.patch
-fbed49fa5ddb0147de4b88f6fa2fb2c5  musl-gcc5-fixes.patch
+97e6eab066fc610e6e3a3b2263a6d5ba  musl-gcc5-fixes.patch
 69f9df7ba92ac3f6c46a4a30a2a5c025  mpd.initd
 dbd301041953a5cf9c2b9de8f23eb6bb  mpd.confd"
 sha256sums="c386eb3d22f98dc993b5ae3c272f969aa7763713483c6800040ebf1791b15851  mpd-0.19.10.tar.xz
 b3a0499cbd3f759612ad94fce00588d97e5e82af0b2bcc560869eb977067a426  underlinking.patch
-8ab0beb4da675380f69ac7102a3c6f1eef41c2984adc7426e87eb534ce9bac93  musl-gcc5-fixes.patch
+f8be8d5f00576e4734d44d3ee2ad2ddd55351c25eee5a545573a264304514db9  musl-gcc5-fixes.patch
 9d842c202c010b7254dc89971649139b81bfec49f11c19bcba8be8712279ba7b  mpd.initd
 32c072f77c23c2cc2dd7c93b65b73015e0587caaf2740856132af32cee623206  mpd.confd"
 sha512sums="037e1ccd69456bd7d388cbcba26de83144816684fb266995b173df960051e84203d3fc7255d7766eee5dbe0629dbb7745b9821664486158d33da4f29189edef8  mpd-0.19.10.tar.xz
 d6a98c5f60a60ddd5fa94b25864f800a6a6fb6ffe114793293d0ccc71bb5d1cdb847ca39fde4128446b55a38be85634106d5d6f2c77ee450f33631b30140781a  underlinking.patch
-da1124b505d2628b4c009fcb3f8dea106017e84c09e1c24c9aa4063007838520fd500e5a48258a78189e12c8b996d920b71060651cdabeb5a7bd01fec01b3400  musl-gcc5-fixes.patch
+5140ad3eb13b5e141e9b9909ad27ae2a08ac7556b3ff23f97a65ae6dc20f3856a0376b85fe8e90eb37b8fbaccd0c1002b4bc788d0abcb7541c95713e1cc98a3a  musl-gcc5-fixes.patch
 8b13530fbfb2bded3cb82202261d57ff824521e5682f07c8f3d0562348aeb303e6c3d72a457043c4d4909d7106d723fe701a12ab66b45b016d3b940acbeb205b  mpd.initd
 41b2467f5b03f5c4dd7003cd5f56f6cfc1f67af7a9aa2538d70360f839625222bdd0c4b04c33e8cd52eeecfc354da3ca22f5aaab8aee357a5774aaf3503594e7  mpd.confd"

main/mpd/musl-gcc5-fixes.patch

+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
+
+also 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
+
+also, volatile in the type is necessary. without that, LTO can break the code.
+
 --- ./src/notify.hxx.orig
 +++ ./src/notify.hxx
 @@ -28,7 +28,7 @@
@@ -5,7 +20,7 @@
  	bool pending;
  
 -#if !defined(WIN32) && !defined(__NetBSD__) && !defined(__BIONIC__)
-+#if !defined(WIN32) && !defined(__NetBSD__) && !defined(__BIONIC__) && defined(__GLIBC__)
++#if defined(__GLIBC__)
  	constexpr
  #endif
  	notify():pending(false) {}
@@ -16,7 +31,7 @@
  
  public:
 -#if defined(__NetBSD__) || defined(__BIONIC__)
-+#if defined(__NetBSD__) || defined(__BIONIC__) || !defined(__GLIBC__)
++#if !defined(__GLIBC__)
  	/* NetBSD's PTHREAD_COND_INITIALIZER is not compatible with
  	   "constexpr" */
  	PosixCond() {
@@ -27,7 +42,7 @@
  
  public:
 -#if defined(__NetBSD__) || defined(__BIONIC__)
-+#if defined(__NetBSD__) || defined(__BIONIC__) || !defined(__GLIBC__)
++#if !defined(__GLIBC__)
  	/* NetBSD's PTHREAD_MUTEX_INITIALIZER is not compatible with
  	   "constexpr" */
  	PosixMutex() {