build: Make sure pkg.m4 is present.

Suggested by John Darrington <john@darrington.wattle.id.au>.

* configure.ac: Forbid occurrences of 'PKG_CHECK_MODULES' in the
  output.

configure.ac

@@ -41,6 +41,10 @@ AC_ARG_ENABLE([daemon],
 guix_localstatedir="`eval echo $localstatedir | sed -e "s|NONE|/usr/local|g"`"
 AC_SUBST([guix_localstatedir])
 
+dnl We require the pkg.m4 set of macros from pkg-config.
+dnl Make sure it's available.
+m4_pattern_forbid([PKG_CHECK_MODULES])
+
 PKG_CHECK_MODULES([GUILE], [guile-2.0 >= 2.0.5])
 AC_PATH_PROG([GUILE], [guile])
 AC_PATH_PROG([GUILD], [guild])