Include the alternative malloc header instead of malloc.h

This commit fixes a build error on systems that have
`malloc_usable_size` but also enable jemalloc via `--with-jemalloc`.
For example, Ubuntu Precise defines `malloc_usable_size` in malloc.h, so
gc.c will include malloc.h.  This definition conflicts with jemalloc's
definition, so the following error occurs:

```
compiling gc.c
compiling hash.c
In file included from gc.c:50:0:
/usr/include/malloc.h:152:15: error: conflicting types for 'malloc_usable_size'
/usr/include/jemalloc/jemalloc.h:45:8: note: previous declaration of 'malloc_usable_size' was here
cc1: warning: unrecognized command line option "-Wno-self-assign" [enabled by default]
cc1: warning: unrecognized command line option "-Wno-constant-logical-operand" [enabled by default]
cc1: warning: unrecognized command line option "-Wno-parentheses-equality" [enabled by default]
cc1: warning: unrecognized command line option "-Wno-tautological-compare" [enabled by default]
```

Since jemalloc always defines `malloc_usable_size`, this patch just
includes the jemalloc header instead of malloc.h if it's available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63992 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -49,7 +49,9 @@
 # endif
 #endif
 #ifdef HAVE_MALLOC_USABLE_SIZE
-# ifdef HAVE_MALLOC_H
+# ifdef RUBY_ALTERNATIVE_MALLOC_HEADER
+#  include RUBY_ALTERNATIVE_MALLOC_HEADER
+# elif HAVE_MALLOC_H
 #  include <malloc.h>
 # elif defined(HAVE_MALLOC_NP_H)
 #  include <malloc_np.h>