luis@seyori:~/sandbox$ cat test.c
#include <stdio.h>

int main()
{
  int i;
  i++;
  int j = i + 2;

  return 0;
}
luis@seyori:~/sandbox$ gcc --version
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

luis@seyori:~/sandbox$ gcc -Werror=declaration-after-statement test.c
test.c: In function ‘main’:
test.c:7: warning: ISO C90 forbids mixed declarations and code
luis@seyori:~/sandbox$ echo $?
0
luis@seyori:~/sandbox$ gcc -Wdeclaration-after-statement test.c
test.c: In function ‘main’:
test.c:7: warning: ISO C90 forbids mixed declarations and code
luis@seyori:~/sandbox$ echo $?
0
luis@seyori:~/sandbox$ i686-w64-mingw32-gcc --version
i686-w64-mingw32-gcc (GCC) 4.5.2 20101018 (prerelease)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

luis@seyori:~/sandbox$ i686-w64-mingw32-gcc -Werror=declaration-after-statement test.c
test.c: In function ‘main’:
test.c:7:3: error: ISO C90 forbids mixed declarations and code
luis@seyori:~/sandbox$ echo $?
1
luis@seyori:~/sandbox$ i686-w64-mingw32-gcc -Wdeclaration-after-statement test.c
test.c: In function ‘main’:
test.c:7:3: warning: ISO C90 forbids mixed declarations and code
luis@seyori:~/sandbox$ echo $?
0
luis@seyori:~/sandbox$