Hello,

There was a build failure on the EL6 platform, reported by the
autobuilder at https://jenkins-fedora-infra.apps.ci.centos.org/job/libabigail/label=EL6/647/console.

The exact root error was:

    ../../src/abg-dwarf-reader.cc:7649: error: ‘R_AARCH64_ABS64’ was not declared in this scope
    ../../src/abg-dwarf-reader.cc:7653: error: ‘R_AARCH64_PREL32’ was not declared in this scope

This is due to the recent patch:

"e687032 Support pre and post v4.19 ksymtabs for Linux kernel modules"

which (rightfully) introduces the use of the R_AARCH64_{ABS64, PREL32}
macros that are defined in the "elf.h" system header.  Unfortunately,
older versions of that system header don't define those macros.

I have thus committed the patch below to have the configure system
detect the presence of these macros and avoid using them if they are not
present.

Cheers,