build: replace ACX_PTHREAD with AX_PTHREAD

On Cygwin, ./configure failed with:

    ./configure: line XXXX: syntax error near unexpected token ACX_PTHREAD([], [AC_MSG_ERROR([pthread is required])])'

Root cause: ACX_PTHREAD is a legacy macro not provided by modern
Autoconf; the maintained macro is AX_PTHREAD (from autoconf-archive).

Changes:
- Replace ACX_PTHREAD with AX_PTHREAD in configure.ac.
- Rely on system autoconf-archive; do not vendor m4/ax_pthread.m4.
- Keep AC_CONFIG_MACRO_DIR([m4]) (harmless even if empty).

Result:
- autoreconf -fi && ./configure && make succeeds on Cygwin x86_64.
- Linux builds continue to work.

Fixes: #2059

configure.ac

@@ -95,7 +95,7 @@ PKG_CHECK_MODULES([curl], [libcurl >= 7.62.0], [],
 PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.22.0], [],
     [AC_MSG_ERROR([sqlite3 3.22.0 or higher is required])])
 
-ACX_PTHREAD([], [AC_MSG_ERROR([pthread is required])])
+AX_PTHREAD
 AS_IF([test "x$PTHREAD_CC" != x], [ CC="$PTHREAD_CC" ])
 
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