Debian-specific kernel feature tests Upstream assumes features based on the kernel version, but some of these features aren't available in the Debian kernel, or were only enabled at a later point. Also, the release number comparison used by upstream breaks with sid's non-numeric release number of "unstable".
This adjust the feature tests to reflect Debian's kernels: * Drop $OSRELEASE tests * Drop ancient RedHat-specific test * Debian's kernel generally lacks CONFIG_BIG_KEYS * Debian's kernel has CONFIG_KEY_DH_OPERATIONS since 4.19~exp1
Use build flags from environment (dpkg-buildflags) Necessary for hardening flags. Also strip -pie/-fPIE from CFLAGS/LDFLAGS when used to link libraries. It conflicts with -fPIC.