how gcc thinks `char' as signed char or unsigned char ?

[PRC]

---------------------------------
int main()
{
	char a = -7;
	
	if( a < -9 )
		printf("a");
	else
		printf("b");
}
---------------------------------
sde-gcc -c a2.c
c:/a2.c: In function `main':
c:/a2.c:6: warning: comparison is always false due to limited range of data type

It may be the reason for this warning that gcc thinks `char' as 'unsigned char' by default.
Can I change the default configuration by modifying some configuration file?
Or this feature can't be changed after gcc has been built?