do not assume GCC for __builtin_setjmp

Namely recent Sun C compiler has this function, and is not a GCC.
Meanwhile the code without RUBY_JMP_BUF assumes GCC.  We have to define
the macro when we detect __builtin_setjmp for non-GCC compilers.

tool/m4/ruby_setjmp_type.m4

@@ -47,6 +47,6 @@ AS_IF([test x$setjmp_prefix:$setjmp_sigmask = xsig:], [
 AC_MSG_RESULT(${setjmp_prefix}setjmp${setjmp_suffix}${setjmp_cast:+\($setjmp_cast\)}${setjmp_sigmask})
 AC_DEFINE_UNQUOTED([RUBY_SETJMP(env)], [${setjmp_prefix}setjmp${setjmp_suffix}($setjmp_cast(env)${setjmp_sigmask})])
 AC_DEFINE_UNQUOTED([RUBY_LONGJMP(env,val)], [${setjmp_prefix}longjmp($setjmp_cast(env),val)])
-AS_IF([test x$setjmp_prefix != x__builtin_], AC_DEFINE_UNQUOTED(RUBY_JMP_BUF, ${setjmp_sigmask+${setjmp_prefix}}jmp_buf))
+AS_IF([test "(" "$GCC" != yes ")" -o x$setjmp_prefix != x__builtin_], AC_DEFINE_UNQUOTED(RUBY_JMP_BUF, ${setjmp_sigmask+${setjmp_prefix}}jmp_buf))
 AS_IF([test x$setjmp_suffix = xex], [AC_DEFINE_UNQUOTED(RUBY_USE_SETJMPEX, 1)])
 ])dnl