configure.ac: don't use xml-config if --with-libxml-prefix is specified to configure

Same behavior for xslt-config.

diff --git a/configure.ac b/configure.ac
index ed04e0c..39d5d4c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,7 +40,8 @@ AC_ARG_PROGRAM          dnl Transforming Program Names When Installing
 AC_PROG_SED
 AC_PROG_AWK
 
-XSTAR_LIB_CHECK([LIBXML], [xml2-config])
+AS_IF([test "x$LIBXML_PREFIX" = x],
+      [XSTAR_LIB_CHECK([LIBXML], [xml2-config])])
 
 AS_IF([test "x$LIBXML_SRCDIR" != x],
       [LIBXML_INCDIR="$LIBXML_SRCDIR/include"])
@@ -56,7 +57,8 @@ AS_IF([test "x$STATIC_LIBS" != xno],
 [LIBXML_LDFLAGS="-L$LIBXML_LIBDIR"]
 
 
-XSTAR_LIB_CHECK([LIBXSLT], [xslt-config])
+AS_IF([test "x$LIBXSLT_PREFIX" = x],
+      [XSTAR_LIB_CHECK([LIBXSLT], [xslt-config])])
 
 AS_IF([test "x$LIBXSLT_SRCDIR" != x],
       [XSLTPROC_PATH="$LIBXSLT_SRCDIR/xsltproc:$PATH"