Build: non-floating-point argument in call to function ‘__builtin_isnan’

Building on Ubuntu 24.04.

>./build 
Current spectre source version 2.6-cli-5-119-g1a1053c-dirty...

Building target: spectre...
INFO:     Enabled spectre_sodium (libsodium).
INFO:     Enabled spectre_color (libtinfo).
INFO:     Enabled spectre_json (libjson-c).
api/c/spectre-marshal.c: In function ‘spectre_marshal_read_json’:
api/c/spectre-marshal.c:1185:14: error: non-floating-point argument in call to function ‘__builtin_isnan’
 1185 |         if (!isnan(defaultType)) {
      |              ^~~~~
api/c/spectre-marshal.c:1187:17: error: non-floating-point argument in call to function ‘__builtin_isnan’
 1187 |             if (isnan(resultType)) {
      |                 ^~~~~