add support for musl 1.1.15 and make it the default

Makefile

@@ -5,7 +5,7 @@ SOURCES = sources
 CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.25.1
 GCC_VER = 5.3.0
-MUSL_VER = 1.1.14
+MUSL_VER = 1.1.15
 GMP_VER = 6.1.0
 MPC_VER = 1.0.3
 MPFR_VER = 3.1.4

hashes/musl-1.1.15.tar.gz.sha1

@@ -0,0 +1 @@
+027c3ae2182fa53c2b554ca98a28dc5cfca4b2c3  musl-1.1.15.tar.gz