reset default of libexecdir

We want to revert to the codings standards here.
https://www.gnu.org/prep/standards/html_node/Directory-Variables.html

By using the default we might avoid differences betweeen bee install and
"make install".

src/beelib.config.sh.in

@@ -309,7 +309,7 @@ function config_verify_builtin_prefixes() {
     : ${EPREFIX:='${PREFIX}'}
     : ${BINDIR:='${EPREFIX}/bin'}
     : ${SBINDIR:='${EPREFIX}/sbin'}
-    : ${LIBEXECDIR:='${EPREFIX}/lib/${PKGNAME}'}
+    : ${LIBEXECDIR:='${EPREFIX}/libexec'}
     : ${SYSCONFDIR:=DEFAULT}
     : ${LOCALSTATEDIR:=DEFAULT}
     : ${SHAREDSTATEDIR:='${LOCALSTATEDIR}'}