xsel: disable check for autoconf vfork AC_FUNC_FORK

* disable check for autoconf AC_FUNC_FORK vfork checking caching to yes.
* AC_FUNC_FORK will create a `#define vfork fork` macro in config.h
* vfork has been mark depercated in macOS 12.
* This macro will mark fork() usage as depercated due to
  config.h being included before system header <unistd.h>.

Re: kfish/xsel#42
Closes: https://trac.macports.org/ticket/63748

x11/xsel/Portfile

@@ -28,6 +28,15 @@ depends_build           port:xorg-libXt
 
 depends_lib             port:xorg-libX11
 
+# cache autoconf vfork check to yes on darwin 21+ (macOS Monterey) disabling
+# the autoconf vfork macro "#define vfork fork" that will result in a build
+# failure due to vfork deprecation warning in macOS Monterey and -Werror
+# https://trac.macports.org/ticket/63748
+# https://github.com/kfish/xsel/issues/42
+if {${os.platform} eq "darwin" && ${os.major} >= 21} {
+    configure.args-append ac_cv_func_vfork=yes
+}
+
 post-destroot {
     set docdir ${destroot}${prefix}/share/doc/${subport}
     xinstall -d ${docdir}