[ruby/prism] Let an embedder set PM_ARENA_INITIAL_SIZE

The arena's first block is 8 KB, sized for a host. An embedder on a
small heap parses short programs (a REPL line uses about 6 KB in two
arenas) and pays two 8 KB blocks for each; with the constant under
#ifndef it can build with -DPM_ARENA_INITIAL_SIZE=2048 instead.

https://github.com/ruby/prism/commit/f7a062efd4

prism/arena.c

@@ -12,8 +12,13 @@
  */
 #define PM_ARENA_BLOCK_SIZE(data_size) (offsetof(pm_arena_block_t, data) + (data_size))
 
-/** Initial block data size: 8 KB. */
+/**
+ * Initial block data size: 8 KB. An embedder on a small heap can define a
+ * smaller one; a parse of a short program then takes blocks of that size.
+ */
+#ifndef PM_ARENA_INITIAL_SIZE
 #define PM_ARENA_INITIAL_SIZE 8192
+#endif
 
 /** Double the block size every this many blocks. */
 #define PM_ARENA_GROWTH_INTERVAL 8