GNU C supports using compound statements as expressions.

check.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char **argv) {
   if (argc < 2) {
      fprintf(stderr, "usage: check x|y\n");
      return 1;
   }

   const int is_x = ({
      int _x;
      if (strcmp(argv[1], "x") == 0)
         _x = 1;
      else
         _x = 0;
      _x;
   });

   printf("is_x = %d\n", is_x);

   return 0;
}