--- pkg.m4.req	2008-01-02 13:23:59.000000000 +0100
+++ pkg.m4	2008-01-02 14:23:25.000000000 +0100
@@ -47,10 +47,12 @@
 # Check to see whether a particular set of modules exists.  Similar
 # to PKG_CHECK_MODULES(), but does not set variables or print errors.
 #
-#
-# Similar to PKG_CHECK_MODULES, make sure that the first instance of
-# this or PKG_CHECK_MODULES is called, or make sure to call
-# PKG_CHECK_EXISTS manually
+# Please remember that m4 expands AC_REQUIRE([PKG_PROG_PKG_CONFIG]) to
+# PKG_PROG_PKG_CONFIG only at the very first occurence in configre.ac;
+# all subsequent occurences expand to nothing.  Consequently, if the
+# first occurence of PKG_CHECK_EXISTS/PKG_CHECK_MODULES in the
+# confugure.ac might be skipped (e.g., it is inside an `if') you have
+# to add an explicit PKG_PROG_PKG_CONFIG (e.g., before that `if').
 # --------------------------------------------------------------
 AC_DEFUN([PKG_CHECK_EXISTS],
 [AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl