[exim] Oddity compilnf exim 4.94

Was O on the first computer I used (Debian buster) but on openSUSE
13.1 (yes I know it is old but lockdown has paused upgrade) i get

...
/bin/sh ../scripts/Configure-os.h
cc -DMACRO_PREDEF macro_predef.c
macro_predef.c: In function ‘options_from_list’:
macro_predef.c:84:1: error: ‘for’ loop initial declarations are only
allowed in C99 mode
for (int i = 0; i < nopt; i++) if (*(s = US opts[i].name) && *s != '*')
^
macro_predef.c:84:1: note: use option -std=c99 or -std=gnu99 to compile
your code
make[1]: *** [macro_predef.o] Error 1
make[1]: Leaving directory `/usr/build/exim-4.94/build-Linux-i386'
make: *** [all] Error 2

So it is using c99 now. Where to I add the option?

==John ff