Fix for automake 1.12+
http://dev.mutt.org/trac/ticket/3579

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

diff -Nura mutt-1.5.21.orig/configure.ac mutt-1.5.21/configure.ac
--- mutt-1.5.21.orig/configure.ac	2013-09-16 11:07:25.699781892 -0300
+++ mutt-1.5.21/configure.ac	2013-09-16 11:07:45.120419960 -0300
@@ -27,7 +27,6 @@
 
 AC_PROG_CC
 AC_ISC_POSIX
-AM_C_PROTOTYPES
 if test "x$U" != "x"; then
   AC_MSG_ERROR(Compiler not ANSI compliant)
 fi