more compile warnings with new factor

[Pádraig Brady]

If I manually undef HAVE_GMP at the top of factor.c, then...

$ make src/factor.o
src/factor.c:154:0: error: macro "__GMP_GNUC_PREREQ" is not used 
[-Werror=unused-macros]

$ make CFLAGS=-m32 src/factor.o
src/factor.c: In function 'factor_using_division':
src/factor.c:755:33: error: variable 'lo' set but not used 
[-Werror=unused-but-set-variable]
src/factor.c: In function 'mulredc':
src/factor.c:945:28: error: variable 'tl' set but not used 
[-Werror=unused-but-set-variable]
src/factor.c: In function 'mulredc2':
src/factor.c:965:28: error: variable 'p0' set but not used 
[-Werror=unused-but-set-variable]
src/factor.c: In function 'prime2_p':
src/factor.c:1323:17: error: variable '_p0' set but not used 
[-Werror=unused-but-set-variable]
src/factor.c: In function 'factor_using_pollard_rho2':
src/factor.c:1579:11: error: variable '_p0' set but not used 
[-Werror=unused-but-set-variable]
src/factor.c: In function 'factor_using_squfof':
src/factor.c:2146:19: error: variable '_p0' set but not used 
[-Werror=unused-but-set-variable]

The above point to redundant processing in umul_ppmm()
(or at least our use of it).

src/factor.c: At top level:
src/factor.c:151:0: error: macro "ASSERT" is not used [-Werror=unused-macros]
src/factor.c:154:0: error: macro "__GMP_GNUC_PREREQ" is not used 
[-Werror=unused-macros]

I'll clean up these tomorrow.

thanks,
Pádraig.