The way the build was fixed for gcc >= 5.3 breaks the build for older
compiler versions on waf. Fix this by ifdef'ing for WAF_BUILD and
checking the compiler version.
This header is used by waf to contain the generated version macros,
particularly using the git hash. For waf it's better to be in a separate
header since it then can keep track of changes on it a trigger
recompilation.
For the make build system, a dummy ap_version.h file has been added in
the missing/ folder so both implementations can co-exist.
Add a cstddef header to allow using std::nullptr_t with those toolchains
that don't provide it. The idea is to make these platforms to use our
wrapper header (see https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html)
and then we add the missing bits to the header.
