configure: Fix to have compiler warnings enabled while building libfips

In commit e42d9f224a4ef2784f8fd43f9f4f5c593a7ddd57 , when the flags
were split between fips and libfips, the warnings flags were
mistakenly applied to both CFLAGS and LDFLAGS of fips. (What was
actually intended was to have the warnings applied to the CFLAGS of
both fips and libfips).