Fixed essentially the same way upstream via:

commit 3df34834189ba45a090f1dcba7d71d23f7b11735
Author: edoapra <edoardo.apra@gmail.com>
Date:   Fri Oct 9 13:36:42 2020 -0700

    fix for clang from Xcode 12

diff -up ga-5.7.2/configure.c99 ga-5.7.2/configure
--- ga-5.7.2/configure.c99	2022-12-12 08:23:27.806493737 +0100
+++ ga-5.7.2/configure	2022-12-12 08:24:00.978163147 +0100
@@ -23022,6 +23022,8 @@ void fix_f_string_for_c(char *s, int len
     }
 }
 
+void $FSUB(void);
+
 int main(int argc, char **argv)
 {
     $FSUB();
diff -up ga-5.7.2/m4/ga_f2c_string.m4.c99 ga-5.7.2/m4/ga_f2c_string.m4
--- ga-5.7.2/m4/ga_f2c_string.m4.c99	2022-12-12 08:23:44.230330055 +0100
+++ ga-5.7.2/m4/ga_f2c_string.m4	2022-12-12 08:24:06.457108545 +0100
@@ -51,6 +51,8 @@ void fix_f_string_for_c(char *s, int len
     }
 }
 
+void $FSUB(void);
+
 int main(int argc, char **argv)
 {
     $FSUB();