Removed -std=c89 option

[veblush]

UPB is not strictly C89 compatible because it uses stdbool.h introduced with C99. The latest version of clang with -Wc99-extensions option throws an error saying it's not allowed with C89. So this PR removes the option.

# cat a.c
#include <stdbool.h>

int main() {
  bool a;
  return 0;
}

# clang a.c -std=c89

# clang a.c -std=c89 -Wc99-extensions
a.c:4:3: warning: '_Bool' is a C99 extension [-Wc99-extensions]
  bool a;
  ^
/usr/lib/llvm-10/lib/clang/10.0.0/include/stdbool.h:15:14: 
  note: expanded from macro 'bool'
#define bool _Bool
             ^
1 warning generated.

# clang a.c -std=c99 -Wc99-extensions

Fixes #212

[haberman]

I want to stay c89 compatible if possible. I may introduce a UPB_BOOL macro that conditionally is defined to bool if C99 is available. But I'll approve this for now to unblock you.

[yang-g]

Thanks for the quick fix!