Fix non-portable use of echo -n

https://github.com/dleidert/chemical-mime/pull/7
--- acinclude.m4.orig	2007-01-02 21:58:02.000000000 -0600
+++ acinclude.m4	2026-03-25 07:57:55.000000000 -0500
@@ -36,8 +36,7 @@
 	[More options, which should be used along with 'xmllint', like e.g. '--nonet'.]
 )
 AC_SUBST([XMLLINT_FLAGS])
-echo -n "checking for optional xmllint options to use... "
-echo $XMLLINT_FLAGS
+printf "checking for optional xmllint options to use... %s\n" "$XMLLINT_FLAGS"
 ]) # MP_PROG_XMLLINT
 
 
@@ -68,7 +67,6 @@
 	[More options, which should be used along with 'xsltproc', like e.g. '--nonet'.]
 )
 AC_SUBST([XSLTPROC_FLAGS])
-echo -n "checking for optional xsltproc options to use... "
-echo $XSLTPROC_FLAGS
+printf "checking for optional xsltproc options to use... %s\n" "$XSLTPROC_FLAGS"
 ]) # MP_PROG_XSLTPROC