Add cxxopts command line parsing library

We'll use this to better manage the fairly large set of command line
options in self-contained objects instead of a scattering of
individual stack variables.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

CMakeLists.txt

@@ -150,6 +150,8 @@ if(NOT MINIMAL_BUILD)
   include(civetweb)
 endif()
 
+include(cxxopts)
+
 # Lpeg
 include(lpeg)
 

cmake/modules/cxxopts.cmake

@@ -0,0 +1,23 @@
+#
+# Copyright (C) 2022 The Falco Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations under the License.
+#
+
+set(CXXOPTS_SRC "${PROJECT_BINARY_DIR}/cxxopts-prefix/src/cxxopts/")
+set(CXXOPTS_INCLUDE_DIR "${CXXOPTS_SRC}/include")
+
+ExternalProject_Add(
+        cxxopts
+        URL "https://github.com/jarro2783/cxxopts/archive/refs/tags/v3.0.0.tar.gz"
+        URL_HASH "SHA256=36f41fa2a46b3c1466613b63f3fa73dc24d912bc90d667147f1e43215a8c6d00"
+	CONFIGURE_COMMAND ""
+	BUILD_COMMAND ""
+	INSTALL_COMMAND "")