cmake: enable hardening compiler flags and fix the reported issues

cmake: enable hardening compiler flags and fix the reported issues

Lets enable some useful flags in order to spot possible issues during
QA on CI (GCC version 6 and higher). Fix warnings uncovered by this new
flags as reported by clang-9 on x86/64:

 ucert.c:158:33: error: comparison of integers of different signs: 'unsigned long' and 'int' [-Werror,-Wsign-compare]
 ucert.c:176:14: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ucert.c:314:18: error: comparison of integers of different signs: '__time_t' (aka 'long') and 'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
 ucert.c:315:18: error: comparison of integers of different signs: '__time_t' (aka 'long') and 'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
 ucert.c:557:17: error: comparison of integers of different signs: '__time_t' (aka 'long') and 'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]

Ref: https://developers.redhat.com/blog/2018/03/21/compiler-and-linker-flags-gcc/
Signed-off-by: Petr Štetiar <ynezz@true.cz>