[gcc r12-4936] include: Allow for our md5.h to defer to the system header

[gcc r12-4936] include: Allow for our md5.h to defer to the system header

[Gerald Pfeifer]

https://gcc.gnu.org/g:44d9d55c6d0e3a1e26427662d30f350a80282634

commit r12-4936-g44d9d55c6d0e3a1e26427662d30f350a80282634
Author: Gerald Pfeifer <gerald@pfeifer.com>
Date:   Fri Nov 5 12:56:07 2021 +0100

    include: Allow for our md5.h to defer to the system header
    
    This came up in the context of libsanitizer, where platform-specific
    support for FreeBSD relies on aspects provided by FreeBSD's own md5.h.
    
    Address this by allowing GCC's md5.h to pull in the system header
    instead, controlled by a new macro USE_SYSTEM_MD5.
    
    2021-11-05  Gerald Pfeifer  <gerald@pfeifer.com>
                Jakub Jelinek  <jakub@redhat.com>
    
    include/
            * md5.h (USE_SYSTEM_MD5): Introduce.

Diff:
---
 include/md5.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/md5.h b/include/md5.h
index 03f7d29afc7..c5bb6076969 100644
--- a/include/md5.h
+++ b/include/md5.h
@@ -21,6 +21,10 @@
 #ifndef _MD5_H
 #define _MD5_H 1
 
+#ifdef USE_SYSTEM_MD5
+#include_next <md5.h>
+#else
+
 #include <stdio.h>
 
 #if defined HAVE_LIMITS_H || _LIBC
@@ -151,4 +155,6 @@ extern void *md5_buffer (const char *buffer, size_t len, void *resblock);
 }
 #endif
 
+#endif // USE_SYSTEM_MD5
+
 #endif