build: use envvars for CFLAGS rather than configparms (LP: #2115734)

Oddly enough, overriding CFLAGS in configparms leads to weird behaviours where target-specific CFLAGS are sometimes being ignored. For instance, some math tests are normally compiled with -mabi=ibmlongdouble on ppc64el but that flag is dropped if CFLAGS is defined in configparms, rather than simply adding that flag to the list.

The glibc documentation doesn't actually talk about CFLAGS when discussion configparms, and the configure script explicitly calls out CFLAGS as an influential env variable, so let's use that to pass flags rather than configparms.