build: don't set default build type and define _GNU_SOURCE
build: don't set default build type and define _GNU_SOURCE

We will let CMake handle default build type. Also remove all custom
CFLAGS and use system ones.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1332067
Signed-off-by: Igor Gnatenko <c869b0bfe2cf1da26c2efd33f6d6d74ddcbbdd2b@gmail.com>

Conan-Kudo/hawkey,rpm-software-management/hawkey,rpm-software-management/hawkey,rpm-software-management/hawkey,Conan-Kudo/hawkey,rpm-software-management/hawkey,Conan-Kudo/hawkey,Conan-Kudo/hawkey