Define the GNULIB_XALLOC_DIE macro

[fweimer-rh]

This avoids an implicit function declaration when building gnulib's xmalloc.c, addressing a build failure with future compiler version.

Solution proposed by Bruno Haible here: Re: xmalloc calling undeclared xalloc_die function

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC