Only define PAGE_SIZE if not already defined.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/sec_mem.c

diff --git a/crypto/sec_mem.c b/crypto/sec_mem.c
index 5bf8baa..a630cbc 100644
--- a/crypto/sec_mem.c
+++ b/crypto/sec_mem.c
@@ -26,7 +26,9 @@
 #define LOCK()      CRYPTO_w_lock(CRYPTO_LOCK_MALLOC)
 #define UNLOCK()    CRYPTO_w_unlock(CRYPTO_LOCK_MALLOC)
 #define CLEAR(p, s) OPENSSL_cleanse(p, s)
-#define PAGE_SIZE    4096
+#ifndef PAGE_SIZE
+# define PAGE_SIZE    4096
+#endif
 
 #ifdef IMPLEMENTED
 size_t secure_mem_used;