build-sys: Do not require xsltproc for installation of man pages

Same reason as found in this commit to systemd:

	commit 4ca39b280fce3c60d2fdecbd478fd9bf7f9d3e64
	Author: Mike Gilbert <[email protected]>
	Date:   Sun Feb 23 11:21:13 2014 -0500

	    configure: Do not require xsltproc for installation of man pages

	    The release tarballs ship with pre-generated man pages, so we do not
	    need xsltproc for a typical end-user build.

	    Developers will probably have xsltproc anyway, but if not they will now
	    encounter a build-time failure instead of an error in configure.
diff --git a/configure.ac b/configure.ac
index 4d9496e..54a52ea 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,6 +36,7 @@
 AC_PROG_SED
 AC_PROG_MKDIR_P
 PKG_PROG_PKG_CONFIG
+AC_PATH_PROG([XSLTPROC], [xsltproc])
 
 
 #####################################################################
@@ -111,10 +112,7 @@
         AS_HELP_STRING([--disable-manpages], [disable manpages @<:@default=enabled@:>@]),
 	[], enable_manpages=yes)
 AM_CONDITIONAL([BUILD_MANPAGES], [test "x$enable_manpages" = "xyes"])
-AC_PATH_PROG([XSLTPROC], [xsltproc], [no])
-AS_IF([test "x$XSLTPROC" = "xno" && test "x$enable_manpages" = "xyes"], [
-	AC_MSG_ERROR([xsltproc command not found, try ./configure --disable-manpages])
-])
+AS_IF(["x$enable_manpages" != "xno"], ["have_manpages=yes"])
 
 AC_ARG_ENABLE([logging],
 	AS_HELP_STRING([--disable-logging], [disable system logging @<:@default=enabled@:>@]),