maint: add --so-version option to release.pl

Since we always update the so version string right before we cut the
tarball, it is more convenient and robust to add this option to the
release script.

maint/release.pl

@@ -14,6 +14,7 @@
 my $arg = 0;
 my $branch = "";
 my $version = "";
+my $so_version = "";
 my $append_commit_id;
 my $root = cwd();
 my $with_autoconf = "";
@@ -122,6 +123,7 @@ sub run_cmd
 GetOptions(
     "branch=s" => \$branch,
     "version=s" => \$version,
+    "so-version=s" => \$so_version,
     "append-commit-id!" => \$append_commit_id,
     "with-autoconf" => \$with_autoconf,
     "with-automake" => \$with_automake,
@@ -208,6 +210,10 @@ sub run_cmd
 my $date = `date`;
 chomp $date;
 system(qq(perl -p -i -e 's/\\[MPICH_RELEASE_DATE_m4\\],\\[unreleased development copy\\]/[MPICH_RELEASE_DATE_m4],[$date]/g' ./maint/version.m4));
+
+if ($so_version) {
+    system(qq(perl -p -i -e 's/\\[libmpi_so_version_m4\\],\\[0:0:0\\]/[libmpi_so_version_m4],[$so_version]/g' ./maint/version.m4));
+}
 # the main version.m4 file will be copied to hydra's version.m4, including the
 # above modifications
 print("done\n");