Fix check for malloc_conf when no library is required

configure.ac

@@ -1303,8 +1303,11 @@ AS_IF([test "x$with_jemalloc" != xno],[
       with_jemalloc=yes
     ],
     [test x$with_jemalloc = xyes && with_jemalloc=no])
-  AC_CHECK_HEADER(jemalloc/jemalloc.h, [
-    AC_DEFINE(RUBY_ALTERNATIVE_MALLOC_HEADER, [<jemalloc/jemalloc.h>])
+  for malloc_header in jemalloc/jemalloc.h jemalloc.h; do
+    AC_CHECK_HEADER($malloc_header, [break], [malloc_header=])
+  done
+  AS_IF([test "$malloc_header" != ""], [
+    AC_DEFINE_UNQUOTED(RUBY_ALTERNATIVE_MALLOC_HEADER, [<$malloc_header>])
   ],
   [test x$with_jemalloc = xyes && with_jemalloc=no])
   AS_IF([test "x$with_jemalloc" != xyes], [
@@ -1313,7 +1316,7 @@ AS_IF([test "x$with_jemalloc" != xno],[
       @%:@ifdef RUBY_ALTERNATIVE_MALLOC_HEADER
       @%:@include RUBY_ALTERNATIVE_MALLOC_HEADER
       @%:@else
-      @%:@include <jemalloc.h>
+      extern const char *malloc_conf;
       @%:@endif], [return !&malloc_conf])],
       [rb_cv_jemalloc_demangle=yes],
       [rb_cv_jemalloc_demangle=no])