config: fix libmpi_abi.so version-info so the intended SONAME is applied

[jsquyres]

Summary

libmpi_abi is currently installed with an SONAME of libmpi_abi.so.0
even though maint/version.m4 sets libmpi_abi_so_version_m4 to
1:0:0 (which should yield libmpi_abi.so.1). The intended
-version-info never reaches libtool, so libtool falls back to its
0:0:0 default. Two independent bugs cause this:

1. Typo. configure.ac referenced the macro as
   libmpi_abi_so_verion_m4 (missing the s in version). That macro
   is undefined, so m4 passes the literal token through — you can see it
   survive verbatim into the generated configure:

   ABI_ABIVERSIONFLAGS="-version-info libmpi_abi_so_verion_m4"
   

2. Missing AC_SUBST. ABI_ABIVERSIONFLAGS was never AC_SUBST'd
   (unlike ABIVERSIONFLAGS), so even the bogus string never reached
   the Makefiles. In the generated Makefile, $(ABI_ABIVERSIONFLAGS)
   expands to empty on the lib_libmpi_abi_la_LDFLAGS line, and libtool
   applies no -version-info → default 0:0:0 → libmpi_abi.so.0.

Fix

• Correct the macro name so it expands to the 1:0:0 already defined in
  maint/version.m4.
• export and AC_SUBST ABI_ABIVERSIONFLAGS, exactly as
  ABIVERSIONFLAGS already is, so the value flows to the link line.

With this change libmpi_abi installs with SONAME libmpi_abi.so.1,
matching the 1:0:0 intent already encoded in maint/version.m4.

Why libmpi_abi.so.1

The point of the standard ABI is that an application can switch backends
without recompiling, which requires every implementation of a given ABI
version to expose the same SONAME. Setting the SONAME to match
MPI_ABI_VERSION (so ABI v1 → libmpi_abi.so.1 everywhere) is what
maint/version.m4's 1:0:0 already intends; this PR just makes it take
effect. See the versioning discussion in mpi-forum/mpi-abi-stubs#28 and
the Open MPI ABI work in open-mpi/ompi#13280. (Open MPI is setting its
libmpi_abi SONAME to .so.1 to match.)

Verification

Verified by structural analogy: the sibling libmpi_so_version_m4 uses
the identical m4_define / expansion idiom in the same version.m4 and
resolves correctly (ABIVERSION = 0:0:0 in the generated Makefile);
this change makes libmpi_abi_so_version_m4 flow the same way. A CI
build with --enable-mpi-abi should confirm the installed
libmpi_abi.so.1 SONAME.

Notes / possible follow-up

This PR intentionally keeps the hard-coded 1:0:0 in maint/version.m4
and only fixes the plumbing so it applies. Deriving the c:r:a triplet
programmatically from MPI_ABI_VERSION/MPI_ABI_SUBVERSION (per the
formula in mpi-abi-stubs#28) would be a larger, separate change and is
left out here.