build: update to autoconf 2.71

This removes warning about deprecated stuff. Notably AC_PROG_CC now
does what AC_PROG_CC_C99 did.

configure.ac

@@ -5,7 +5,7 @@
 ### Base configuration
 
 # Configure autoconf
-AC_PREREQ([2.69])
+AC_PREREQ([2.71])
 
 AC_INIT([lldpd],
         [m4_esyscmd_s([./get-version])],
@@ -14,7 +14,7 @@ AC_INIT([lldpd],
         [https://lldpd.github.io/])
 
 AC_CONFIG_SRCDIR([src/log.c])
-AC_CONFIG_HEADER([config.h])
+AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_FILES([Makefile
                  src/Makefile
                  src/compat/Makefile
@@ -44,7 +44,6 @@ gl_LD_VERSION_SCRIPT
 
 # Checks for programs.
 AC_PROG_CC
-AC_PROG_CC_C99
 if test x"$ac_cv_prog_cc_c99" = x"no"; then
   AC_MSG_FAILURE([*** C99 support is mandatory])
 fi

m4/alignof.m4

@@ -3,10 +3,9 @@
 #
 AC_DEFUN([lldp_CHECK_ALIGNOF],[
   AC_CACHE_CHECK([whether compiler understands __alignof__], lldp_cv_check_alignof, [
-    AC_TRY_COMPILE([],
-	[ return __alignof__(long); ],
-	[ lldp_cv_check_alignof="yes" ],
-	[ lldp_cv_check_alignof="no" ])
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[ return __alignof__(long); ]])],
+                      [ lldp_cv_check_alignof="yes" ],
+                      [ lldp_cv_check_alignof="no" ])
   ])
   if test x"$lldp_cv_check_alignof" = x"yes"; then
      AC_DEFINE([HAVE_ALIGNOF], [1], [Define if __alignof__ operator is available])

m4/ax_cflags_gcc_option.m4

@@ -81,15 +81,16 @@ AS_VAR_PUSHDEF([VAR],[ac_cv_cflags_gcc_option_$1])dnl
 AC_CACHE_CHECK([FLAGS for gcc m4_ifval($1,$1,-option)],
 VAR,[AS_VAR_SET([VAR],["no, unknown"])
  AC_LANG_SAVE
- AC_LANG_C
+ AC_LANG([C])
  ac_save_[]FLAGS="$[]FLAGS"
 for ac_arg dnl
 in "-pedantic -Werror % m4_ifval($1,$1,-option)"  dnl   GCC
    "-pedantic % m4_ifval($1,$1,-option) %% no, obsolete"  dnl new GCC
    #
 do FLAGS="$ac_save_[]FLAGS $[]m4_ifval($2,$2,) "`echo $ac_arg | sed -e 's,%%.*,,' -e 's,%,,'`
-   AC_TRY_COMPILE([],[return 0;],
-   [AS_VAR_SET([VAR],[`echo $ac_arg | sed -e 's,.*% *,,'`]) ; break])
+   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[return 0;]])],
+                     [AS_VAR_SET([VAR],[`echo $ac_arg | sed -e 's,.*% *,,'`]) ; break],
+                     [])
 done
  FLAGS="$ac_save_[]FLAGS"
  AC_LANG_RESTORE

m4/ax_ld_check_flag.m4

@@ -50,7 +50,7 @@
 #serial 6
 
 AC_DEFUN([AX_LDFLAGS_OPTION],[
-  AC_PREREQ([2.61])
+  AC_PREREQ([2.71])
   AC_REQUIRE([AC_PROG_SED])
 
   flag=`echo "$1" | $SED 'y% .=/+-(){}<>:*,%_______________%'`
@@ -59,7 +59,7 @@ AC_DEFUN([AX_LDFLAGS_OPTION],[
     [ax_cv_ld_check_flag_$flag],[
 
     AC_LANG_SAVE
-    AC_LANG_C
+    AC_LANG([C])
 
     save_LDFLAGS="$LDFLAGS"
     LDFLAGS="-Werror $LDFLAGS $[]m4_ifval($2,$2,) $1"