build: Abort if there is no support for the required C99 extensions

debian/changelog

@@ -15,6 +15,7 @@ dpkg (1.15.5) UNRELEASED; urgency=low
     Thanks to Jonathan Nieder <jrnieder@gmail.com>.
   * On ‘dpkg-trigger --help’ print the default admindir instead of the one
     passed on the command line.
+  * Abort on configure if the required C99 extensions are not supported.
 
   [ Raphaël Hertzog ]
   * Add versioned dependency on base-files (>= 5.0.0) to dpkg-dev to ensure

m4/compiler.m4

@@ -102,6 +102,7 @@ AS_IF([test "x$dpkg_cv_c99" = "xyes"],
 		 done])
 	 AS_IF([test "x$dpkg_cv_c99_arg" != "xnone"],
 	       [CC="$CC $dpkg_cv_c99_arg"
-		AC_DEFINE([HAVE_C99], 1)])])[]dnl
+		AC_DEFINE([HAVE_C99], 1)],
+	       [AC_MSG_ERROR([unsupported required C99 extensions])])])[]dnl
 ])# DPKG_C_C99