diff --git a/tp/Texinfo/XS/configure.ac b/tp/Texinfo/XS/configure.ac
index b9f9fcf658..ec369d3e7b 100644
--- a/tp/Texinfo/XS/configure.ac
+++ b/tp/Texinfo/XS/configure.ac
@@ -147,9 +147,15 @@ AM_CONDITIONAL([HAVE_ICONV],
                [test "x$am_func_iconv" = "xyes"])
 
 
-# Do not include Perl configuration values when outputting these variables
+# Do not include Perl configuration values when outputting these variables,
+# as advised in the automake manual for CFLAGS and LDFLAGS, which could be
+# reset by the user.  Here PERL_EXT_CFLAGS and PERL_EXT_LDFLAGS play the
+# same role as CFLAGS and LDFLAGS.  The Perl configuration values are then
+# readded in Makefile.am.
+# For CPPFLAGS we do not follow the same rule because we want to keep the
+# flags added by gnulib macros, as for libunistring there is no separate
+# CPPFLAGS-like variable, the main CPPFLAGS is modified.
 CFLAGS=$PERL_EXT_CFLAGS
-CPPFLAGS=$PERL_EXT_CPPFLAGS
 LDFLAGS=$PERL_EXT_LDFLAGS
 
 AC_CONFIG_FILES([Makefile gnulib/lib/Makefile])