make: allow default distro definition of CFLAGS
make: allow default distro definition of CFLAGS

make non-required CFLAGS optional if none are defined from the env
like in distro builds.

ZerBea/hcxtools,RealEnder/hcxtools,ZerBea/hcxtools,RealEnder/hcxtools,ZerBea/hcxtools,RealEnder/hcxtools