build: replace obsolete autoconf macros

autoconf 2.70 emits deprecation warnings for AC_PROG_LIBTOOL,
AC_TRY_LINK and AC_TRY_COMPILE and tells the user to run autoupdate.
Apply the standard replacements so a fresh autogen.sh is quiet:

* AC_PROG_LIBTOOL -> LT_INIT (libtool 2 spelling).
* AC_TRY_LINK([prologue], [body], ...) ->
  AC_LINK_IFELSE([AC_LANG_PROGRAM([prologue], [body])], ...) in
  AU_CHECK_LIB_SONAME.
* AC_TRY_COMPILE([prologue], [body], ...) ->
  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([prologue], [body])], ...) in
  AU_HAVE_GNU_ERRNO_H.

Behaviour is unchanged: SONAME detection still picks up
libusb-1.0.so.0, the GNU errno.h probe still detects
program_invocation_name, and the resulting library and examples
build cleanly.

Closes: #28
Assisted-by: Claude:claude-opus-4-7

Author: Youw

configure.ac

@@ -6,7 +6,7 @@ AC_CONFIG_MACRO_DIR([m4])
 
 AC_PREREQ([2.50])
 AC_PROG_CC
-AC_PROG_LIBTOOL
+LT_INIT
 AC_C_INLINE
 AM_PROG_CC_C_O
 

m4/au_check_lib_soname.m4

@@ -21,11 +21,11 @@ AC_DEFUN([AU_CHECK_LIB_SONAME], [
           AU_CHECK_LIB_SONAME_LIBS="$LIBS"
           LIBS="$LIBS $7 -l$2"
           shrext_regexp=`echo "$shrext_cmds" | sed 's/\./\\\\./'`
-          AC_TRY_LINK([
+          AC_LINK_IFELSE([AC_LANG_PROGRAM([[
 void libusb_close(void *);
-], [
+]], [[
 libusb_close((void*)0);
-],
+]])],
               [AS_VAR_SET([ac_Lib_SONAME], [`objdump -p conftest$ac_exeext | sed -n 's/@<:@ \t@:>@\+NEEDED@<:@ \t@:>@\+\('lib[$2]$shrext_regexp'.*\)$/\1/p'`])])
           LIBS="$AU_CHECK_LIB_SONAME_LIBS"
           AS_IF([test x"$ac_Lib_SONAME" = x ],

m4/au_have_gnu_errno.m4

@@ -5,10 +5,10 @@ m4_pattern_allow([^AU_])
 # Check for GNU extensions of errno.h.
 AC_DEFUN([AU_HAVE_GNU_ERRNO_H], [
   AC_CACHE_CHECK([for GNU extensions of errno.h], [ac_cv_have_gnu_errno_h],[
-    AC_TRY_COMPILE([
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 #define _GNU_SOURCE
 #include <errno.h>
-    ], [char *testvar = program_invocation_name;],
+    ]], [[char *testvar = program_invocation_name;]])],
     [ac_cv_have_gnu_errno_h=yes], [ac_cv_have_gnu_errno_h=no])])
   AS_IF([test x"$ac_cv_have_gnu_errno_h" = x"yes"], [
     AC_DEFINE([HAVE_GNU_ERRNO_H], [1], [Define to 1 if you have the <errno.h> header file with GNU extensions.])