Personally, I prefer the bit-field syntax, and without macros since my flags are almost always inside structs anyway. However, if you insist on writing assembler in C, here's how:
/* We need to indirect the macro call so that the pair of arguments get expanded */
#define BITSET_(f,i) do{f|= 1<<(i);}while(0)
#define BITCLR_(f,i) do{f&=~(1<<(i));}while(0)
#define BITCHK_(f,i) ((f)&(1<<(i)))
#define BITSET(fi) BITSET_(fi)
#define BITCLR(fi) BITCLR_(fi)
#define BITCHK(fi) BITCHK_(fi)
/* Define the flag container and bit number as per OP */
#define poweron flags1,0
#define warnuser flags7,4
/* Sample uses */
BITSET(poweron);
BITCLR(warnuser);
/* Since BITCHK expands to a parenthesized expression, I can get away with
* leaving out the parentheses in the if statement. Not saying that's a good
* idea or anything.
*/
if BITCHK(poweron) BITSET(warnuser);
If you have gcc, you can verify this with gcc -E flag_macros.c