Add m4/ax_check_capsicum.m4 for detecting Capsicum support.

The file was loaded from this web page:
https://github.com/google/capsicum-test/blob/dev/autoconf/m4/ax_check_capsicum.m4

Thanks to Loganaden Velvindron for pointing it out for me.

m4/ax_check_capsicum.m4

@@ -0,0 +1,86 @@
+# -*- Autoconf -*-
+
+# SYNOPSIS
+#
+#   AX_CHECK_CAPSICUM([action-if-found[, action-if-not-found]])
+#
+# DESCRIPTION
+#
+#   This macro searches for an installed Capsicum library, and if found:
+#    - calls one of AC_DEFINE([HAVE_CAPSICUM_SYS_CAPSICUM_H]) or
+#      AC_DEFINE([HAVE_CAPSICUM_SYS_CAPABILITY_H])
+#    - sets CAPSICUM_LIB to the -l option needed to link Capsicum support.
+#
+#   If either the header file or the library is not found,
+#   shell commands 'action-if-not-found' is run.
+#
+#   If both header file and library are found, shell commands
+#   'action-if-found' is run. If 'action-if-found' is not specified, the
+#   default action:
+#    - calls AC_DEFINE(HAVE_CAPSICUM)
+#    - prepends ${CAPSICUM_LIB} to LIBS.
+#
+#   You should use autoheader to include a definition for the symbols above
+#   in a config.h file.
+#
+#   Sample usage in a C/C++ source is as follows:
+#
+#     #ifdef HAVE_CAPSICUM
+#     # ifdef HAVE_CAPSICUM_SYS_CAPSICUM_H
+#     #  include <sys/capsicum.h>
+#     # else
+#     #  ifdef HAVE_CAPSICUM_SYS_CAPABILITY_H
+#     #   include <sys/capability.h>
+#     #  endif
+#     # endif
+#     #endif /* HAVE_CAPSICUM */
+#
+# LICENSE
+#
+#   Copyright (c) 2014 Google Inc.
+#
+#   Copying and distribution of this file, with or without modification,
+#   are permitted in any medium without royalty provided the copyright
+#   notice and this notice are preserved.  This file is offered as-is,
+#   without any warranty.
+
+AU_ALIAS([CHECK_CAPSICUM], [AX_CHECK_CAPSICUM])
+AC_DEFUN([AX_CHECK_CAPSICUM],
+[AC_CHECK_HEADERS([sys/capability.h sys/capsicum.h])
+capsicum_hdrfound=false
+# If <sys/capsicum.h> exists (Linux, FreeBSD>=11.x), assume it is the correct header.
+if test "x$ac_cv_header_sys_capsicum_h" = "xyes" ; then
+   AC_DEFINE([HAVE_CAPSICUM_SYS_CAPSICUM_H],[],[Capsicum functions declared in <sys/capsicum.h>])
+   capsicum_hdrfound=true
+elif test "x$ac_cv_header_sys_capability_h" = "xyes" ; then
+   # Just <sys/capability.h>; on FreeBSD 10.x this covers Capsicum, but on Linux it
+   # describes POSIX.1e capabilities.  So check it declares cap_rights_limit.
+   AC_CHECK_DECL([cap_rights_limit],
+                  [AC_DEFINE([HAVE_CAPSICUM_SYS_CAPABILITY_H],[],[Capsicum functions declared in <sys/capability.h>])
+                   capsicum_hdrfound=true],[],
+                 [#include <sys/capability.h>])
+fi
+
+AC_LANG_PUSH([C])
+# FreeBSD >= 10.x has Capsicum functions in libc
+capsicum_libfound=false
+AC_LINK_IFELSE([AC_LANG_CALL([], [cap_rights_limit])],
+               [capsicum_libfound=true],[])
+# Linux has Capsicum functions in libcaprights
+AC_CHECK_LIB([caprights],[cap_rights_limit],
+             [AC_SUBST([CAPSICUM_LIB],[-lcaprights])
+              capsicum_libfound=true],[])
+AC_LANG_POP([C])
+
+if test "$capsicum_hdrfound" = "true" && test "$capsicum_libfound" = "true"
+then
+    # If both library and header were found, action-if-found
+    m4_ifblank([$1],[
+                LIBS="${CAPSICUM_LIB} $LIBS"
+                AC_DEFINE([HAVE_CAPSICUM],[],[Capsicum library available])])
+else
+    # If either header or library was not found, action-if-not-found
+    m4_default([$2],[AC_MSG_WARN([Capsicum support not found])])
+fi])
+
+