Fixed building with musl libc (ticket #685).

src/os/unix/ngx_user.c

diff --git a/src/os/unix/ngx_user.c b/src/os/unix/ngx_user.c
index 7a71203..3491f1c 100644
--- a/src/os/unix/ngx_user.c
+++ b/src/os/unix/ngx_user.c
@@ -31,8 +31,10 @@
     struct crypt_data   cd;
 
     cd.initialized = 0;
+#ifdef __GLIBC__
     /* work around the glibc bug */
     cd.current_salt[0] = ~salt[0];
+#endif
 
     value = crypt_r((char *) key, (char *) salt, &cd);