configure: provide surrogates for possibly missing libbpf_version.h

configure: provide surrogates for possibly missing libbpf_version.h

Old libbpf library versions (< 0.7.x) may not have the libbpf_version.h
header packaged. This header would provide LIBBPF_MAJOR_VERSION and
LIBBPF_MINOR_VERSION which are then missing to control conditional
compilation in some source files.

Provide surrogates for these defines via CFLAGS that are derived from
the LIBBPF_VERSION determined with $(${PKG_CONFIG} libbpf --modversion).

Signed-off-by: Stefan Mätje <stefan.maetje@esd.eu>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>