build: ensure that missing Autoconf macros are diagnosed

* configure.ac: Catch missing PKG_* macros.
* Makefile.am (ACLOCAL_AMFLAGS): New.
Put macro files in build-aux.

Makefile.am

@@ -12,6 +12,7 @@
 ## The complete GNU General Public Licence Notice can be found as the
 ## `NOTICE' file in the root directory.
 
+ACLOCAL_AMFLAGS = -I build-aux
 SUBDIRS = src tests man
 EXTRA_DIST = bootstrap NOTICE
 

configure.ac

@@ -12,6 +12,8 @@
 ## The complete GNU General Public Licence Notice can be found as the
 ## `NOTICE' file in the root directory.
 
+m4_pattern_forbid([^PKG_])
+
 ## -------------------------------------
 ## Project
 AC_INIT([MonoBURG], [1.0.6a])