Do not hardcode warning and optimisation flags, otherwise the
main options (i.e. DBG, CPPFLAGS) will not work as expected.
You can still provide specific default by using DBG?=<value>.
Doing so leaves the opportunity to override the setting from the
commandline, while the default value from the build system is
then ignored for that particular package.
When crosscompiling, and using build.sh, adding -V DBG=<value> has
this same effect as make DBG=<value>.