Do not provide funopen() on musl

Fixes: https://bugs.debian.org/818246

configure.ac

@@ -51,7 +51,11 @@ AS_CASE([$host_os],
     AC_SEARCH_LIBS([clock_gettime], [rt], [CLOCK_GETTIME_LIBS="-lrt"])
     AC_SUBST([CLOCK_GETTIME_LIBS])
     LIBS="$saved_LIBS"
-  ]
+  ],
+  [*-musl], [
+    # Upstream refuses to define this, we will do it ourselves then.
+    AC_DEFINE([__MUSL__], [1], [Define to 1 if we are building for musl])
+  ],
 )
 
 # Checks for header files.