libelf: Add ELF compression types and defines to libelf.h for older glibc.

Older glibc elf.h might not define the new ELF compression defines and
types. If not just define them in libelf.h directly to make the libelf
headers work on older glibc systems.

Also include a testcase to check the libelf headers build against the
system elf.h.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810885



Signed-off-by: Mark Wielaard <mjw@redhat.com>