The fix proposed by rt46520 works correctly.

However, I have mixed feelings about introducing workarounds to cater
for external software doing things it apparently has no reason to do,
like defining ISC_BUFFER_USEINLINE on its own.  If we consider dnsperf,
it seems the compiler was very clearly warning about this before commit
5c76f3664c broke compilation for good:

    $ make
    gcc -g -O2 -I/tmp/bind/include -D_REENTRANT -D_GNU_SOURCE -DPACKAGE_NAME=\"\" -DPACKAGE_TARNAME=\"\" -DPACKAGE_VERSION=\"\" -DPACKAGE_STRING=\"\" -DPACKAGE_BUGREPORT=\"\" -DHAVE_LIBNSL=1 -DHAVE_PTHREAD=1 -pthread -c dnsperf.c
    In file included from dnsperf.c:55:0:
    /tmp/bind/include/isc/buffer.h:110:0: warning: "ISC_BUFFER_USEINLINE" redefined
     #define ISC_BUFFER_USEINLINE 1
     
    dnsperf.c:53:0: note: this is the location of the previous definition
     #define ISC_BUFFER_USEINLINE
     
    gcc -g -O2 -I/tmp/bind/include -D_REENTRANT -D_GNU_SOURCE -DPACKAGE_NAME=\"\" -DPACKAGE_TARNAME=\"\" -DPACKAGE_VERSION=\"\" -DPACKAGE_STRING=\"\" -DPACKAGE_BUGREPORT=\"\" -DHAVE_LIBNSL=1 -DHAVE_PTHREAD=1 -pthread -c datafile.c
    In file included from datafile.c:26:0:
    /tmp/bind/include/isc/buffer.h:110:0: warning: "ISC_BUFFER_USEINLINE" redefined
     #define ISC_BUFFER_USEINLINE 1
     
    datafile.c:24:0: note: this is the location of the previous definition
     #define ISC_BUFFER_USEINLINE
     
    gcc -g -O2 -I/tmp/bind/include -D_REENTRANT -D_GNU_SOURCE -DPACKAGE_NAME=\"\" -DPACKAGE_TARNAME=\"\" -DPACKAGE_VERSION=\"\" -DPACKAGE_STRING=\"\" -DPACKAGE_BUGREPORT=\"\" -DHAVE_LIBNSL=1 -DHAVE_PTHREAD=1 -pthread -c dns.c
    In file included from dns.c:44:0:
    /tmp/bind/include/isc/buffer.h:110:0: warning: "ISC_BUFFER_USEINLINE" redefined
     #define ISC_BUFFER_USEINLINE 1
     
    dns.c:41:0: note: this is the location of the previous definition
     #define ISC_BUFFER_USEINLINE

Thus, I would rather report this upstream (possibly fixing the dnsperf
copy in contrib/) than add a configure option to BIND as a workaround.
Am I missing something?