build: Error out if the pkg-config m4 macros are missing on autoreconf

Reported-by: Cyril Brulebois <kibi@debian.org>

m4/dpkg-libs.m4

@@ -54,6 +54,7 @@ AC_DEFUN([DPKG_LIB_BZ2], [
 # Check for selinux library.
 AC_DEFUN([DPKG_LIB_SELINUX], [
 AC_REQUIRE([PKG_PROG_PKG_CONFIG])
+m4_ifndef([PKG_PROG_PKG_CONFIG], [m4_fatal([missing pkg-config macros])])
 AC_ARG_VAR([SELINUX_LIBS], [linker flags for selinux library])dnl
 AC_ARG_WITH(selinux,
 	AS_HELP_STRING([--with-selinux],