Makefile: Obey generic CPPFLAGS/CFLAGS/LDFLAGS from the env.
Most Linux packaging systems provide generic CPPFLAGS/CFLAGS/LDFLAGS environement variables that should be used by every package. These flags are generic and mostly contain CPU architecture, build reproducibility and security options. Not obeying them generally leads to problems with the package.
Note that CMake does not support CPPFLAGS[1] so we have to coalesce it into CFLAGS while invoking it.