Better support for recent Botan (configure, warnings)

Recent versions of Botan display some warnings:

In file included from botan_hash.cc:14:
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/lookup.h:35:1: warning: this header is deprecated [-W#pragma-messages]
BOTAN_DEPRECATED_HEADER(lookup.h)
^
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/compiler.h:104:42: note: expanded from macro 'BOTAN_DEPRECATED_HEADER'
    #define BOTAN_DEPRECATED_HEADER(hdr) _Pragma("message \"this header is deprecated\"")
                                         ^
<scratch space>:214:2: note: expanded from here
 message "this header is deprecated"
 ^
1 warning generated.
In file included from botan_hmac.cc:14:
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/hmac.h:14:1: warning: this header will be made internal in the future [-W#pragma-messages]
BOTAN_FUTURE_INTERNAL_HEADER(hmac.h)
^
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/compiler.h:107:49: note: expanded from macro 'BOTAN_FUTURE_INTERNAL_HEADER'
      #define BOTAN_FUTURE_INTERNAL_HEADER(hdr) _Pragma("message \"this header will be made internal in the future\"")
                                                ^
<scratch space>:214:2: note: expanded from here
 message "this header will be made internal in the future"
 ^
In file included from botan_hmac.cc:15:
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/lookup.h:35:1: warning: this header is deprecated [-W#pragma-messages]
BOTAN_DEPRECATED_HEADER(lookup.h)
^
/usr/local/Cellar/botan/2.12.1/include/botan-2/botan/compiler.h:104:42: note: expanded from macro 'BOTAN_DEPRECATED_HEADER'
    #define BOTAN_DEPRECATED_HEADER(hdr) _Pragma("message \"this header is deprecated\"")
                                         ^
<scratch space>:216:2: note: expanded from here
 message "this header is deprecated"
 ^
2 warnings generated.

BTW these warnings are only displayed: the quote is from a --with-werror build.