build: drop check for ANSI compiler

Using automake.git (will become 1.12 someday), I got this error:

configure.ac:90: error: automatic de-ANSI-fication support has been removed
/usr/local/share/aclocal-1.11a/protos.m4:13: AM_C_PROTOTYPES is expanded from...
configure.ac:90: the top level
autom4te: /usr/bin/m4 failed with exit status: 1

In short, pre-C89 compilers are no longer a viable portability
target.  Besides, our code base already requires C99, so worrying
about pre-C89 seems pointless.

* configure.ac (AM_C_PROTOTYPES): Drop, since newer automake no
longer provides it.

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 
-dnl Copyright (C) 2005-2011 Red Hat, Inc.
+dnl Copyright (C) 2005-2012 Red Hat, Inc.
 dnl See COPYING.LIB for the License of this software
 
 AC_INIT([libvirt], [0.9.8], [libvir-list@redhat.com], [], [http://libvirt.org])
@@ -86,10 +86,6 @@ gl_INIT
 
 AC_TYPE_UID_T
 
-dnl Make sure we have an ANSI compiler
-AM_C_PROTOTYPES
-test "x$U" != "x" && AC_MSG_ERROR([Compiler not ANSI compliant])
-
 dnl Support building Win32 DLLs (must appear *before* AM_PROG_LIBTOOL)
 AC_LIBTOOL_WIN32_DLL