Configure test fails to add -std=gnu99 on Fedora Rawhide

On Fedora Rawhide (F32) I'm seeing the following:

gcc -fPIC -g -O2  -pthread -fobjc-exceptions -fno-strict-aliasing -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -std=c99 -DHAVE_CONFIG_H -Wall  -I/builddir/build/BUILD/openvpn-auth-ldap-auth-ldap-2.0.4  -I/builddir/build/BUILD/openvpn-auth-ldap-auth-ldap-2.0.4 -c auth-ldap.m -o auth-ldap.o -I. -I../src -I.. -I../src -I. -I../tests -I../tests
In file included from ./TRVPNPlugin.h:46,
                 from auth-ldap.m:44:
./TRString.h:50:58: error: unknown type name 'va_list'
   50 | - (id) initWithFormat: (const char *) format arguments: (va_list) arguments;
      |                                                          ^~~~~~~

This can be fixed by adding -std=gnu99 which configure is adding on other versions. The configure check for ISO C99 "succeeds" on F32 with:

configure:2997: checking for gcc option to accept ISO C99
configure:3146: gcc  -c -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fPIC  conftest.c >&5
conftest.c: In function 'test_varargs':
conftest.c:80:9: warning: variable 'fnumber' set but not used [-Wunused-but-set-variable]
   80 |   float fnumber;
      |         ^~~~~~~
conftest.c:79:7: warning: variable 'number' set but not used [-Wunused-but-set-variable]
   79 |   int number;
      |       ^~~~~~
conftest.c:78:15: warning: variable 'str' set but not used [-Wunused-but-set-variable]
   78 |   const char *str;
      |               ^~~
configure:3146: $? = 0
configure:3159: result: none needed

This is with gcc 9.2.1

threerings / openvpn-auth-ldap

Configure test fails to add -std=gnu99 on Fedora Rawhide #78