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.