configure: Add new --enable-install-elfh option.

We explicitly test (with system-elf-libelf) that our include headers
work with the system elf.h header. But it might be helpful to install
the elf.h file for a private install. Our elf.h header really is just
a copy of the latest glibc elf.h. But it might be newer and include
more constants than the system installed elf.h.

Add a new configure option --enable-install-elfh to install elf.h.
But warn when it is enabled for the default /usr or /usr/local prefix
because it might clash with the glibc/system elf.h header in that case.

Signed-off-by: Mark Wielaard <mark@klomp.org>