Minor autoconf cleanup and documentation update

INSTALL

@@ -9,13 +9,23 @@ Default location for configuration file is /usr/local/etc/bird.conf and
 for control socket is /usr/local/var/run/bird.ctl . You can change that
 by --sysconfdir and --localstatedir configure options.
 
+To compile current development BIRD source code from Git repository, you
+also need Git (to download the source code) and Autoconf (to generate
+the configure script and associated files using 'autoreconf' tool):
+
+$ git clone git://git.nic.cz/bird.git
+$ cd bird
+$ autoreconf
+
+Then continue as in usual installation above.
+
 
 Requirements
 ============
 
 For compiling BIRD you need these programs and libraries:
 
- - GNU C Compiler
+ - GNU C Compiler (or LLVM Clang)
  - GNU Make
  - GNU Bison
  - GNU M4

configure.ac

@@ -241,25 +241,7 @@ case $sysdesc in
 esac
 
 AC_CHECK_HEADERS_ONCE([alloca.h syslog.h])
-
-AC_MSG_CHECKING(whether 'struct sockaddr' has sa_len)
-AC_COMPILE_IFELSE([
-  AC_LANG_PROGRAM(
-    [[
-      #include <sys/types.h>
-      #include <sys/socket.h>
-    ]],
-    [[
-      static struct sockaddr sa;
-      int i = sizeof(sa.sa_len);
-    ]]
-  )],
-  [
-    AC_MSG_RESULT(yes)
-    AC_DEFINE(HAVE_SIN_LEN,,sin_len)
-  ],
-  [AC_MSG_RESULT(no)]
-)
+AC_CHECK_MEMBERS([struct sockaddr.sa_len], [], [], [#include <sys/socket.h>])
 
 AC_C_BIGENDIAN(
   [AC_DEFINE([CPU_BIG_ENDIAN], [1], [Define to 1 if cpu is big endian])],

sysdep/unix/io.c

@@ -516,7 +516,7 @@ static inline void
 sockaddr_fill4(struct sockaddr_in *sa, ip_addr a, uint port)
 {
   memset(sa, 0, sizeof(struct sockaddr_in));
-#ifdef HAVE_SIN_LEN
+#ifdef HAVE_STRUCT_SOCKADDR_SA_LEN
   sa->sin_len = sizeof(struct sockaddr_in);
 #endif
   sa->sin_family = AF_INET;