build: require libtool 2.4.2

Every system we support has 2.4.6 available, except for OpenBSD, which
currently ships with 2.4.2 (released 2011). For now, set our minimum
required version to that.

After a 7 year hitus, 2.4.7 has also very recently been released:
https://savannah.gnu.org/forum/forum.php?forum_id=10139.

Partially motivated by comments in #24615.

See also: https://repology.org/project/libtool/versions

configure.ac

@@ -96,10 +96,8 @@ fi
 AC_PROG_OBJCXX
 ])
 
-dnl Since libtool 1.5.2 (released 2004-01-25), on Linux libtool no longer
+dnl OpenBSD ships with 2.4.2
-dnl sets RPATH for any directories in the dynamic linker search path.
+LT_PREREQ([2.4.2])
-dnl See more: https://wiki.debian.org/RpathIssue
-LT_PREREQ([1.5.2])
 dnl Libtool init checks.
 LT_INIT([pic-only win32-dll])