CMake: cxxopts: support building with distro-packaged dependency

Many linux distros (e.g., Fedora) strongly discourage bundling or
vendoring dependencies. Many of them also include a `cxxopts-devel`
package. This patch adds support for un-vendoring `cxxopts` by:

   1. `libs/cxxopts` vendored submodule build conditional on
      `YOSYS_WITH_PKG_DEPS == OFF`

   2. use implicit `<cxxopts.hpp>` instead of explicit hard-coded
      path to vendored header file

   3. conditionally provide additional implicit include path to
      `cxxopts.hpp` and library dependency for `kernel/CMakeLists.txt`

Note that the current existing behavior remains the default.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>

kernel/CMakeLists.txt

@@ -89,9 +89,10 @@ yosys_core(kernel
 	yw.cc
 	yw.h
 	INCLUDE_DIRS
+		$<$<NOT:$<BOOL:${YOSYS_WITH_PKG_DEPS}>>:${CMAKE_CURRENT_SOURCE_DIR}/libs/cxxopts/include/>
 		${pybind11_INCLUDE_DIR}
 	LIBRARIES
-		cxxopts
+		$<$<NOT:$<BOOL:${YOSYS_WITH_PKG_DEPS}>>:cxxopts>
 		$<${YOSYS_ENABLE_PLUGINS}:${Dlfcn_LIBRARIES}>
 		$<${YOSYS_ENABLE_ZLIB}:PkgConfig::zlib>
 		$<${YOSYS_ENABLE_READLINE}:PkgConfig::readline>

kernel/driver.cc

@@ -21,7 +21,7 @@
 #include "kernel/hashlib.h"
 #include "libs/sha1/sha1.h"
 #define CXXOPTS_VECTOR_DELIMITER '\0'
-#include "libs/cxxopts/include/cxxopts.hpp"
+#include <cxxopts.hpp>
 #include <iostream>
 #include <chrono>
 

libs/CMakeLists.txt

@@ -1,5 +1,7 @@
 add_subdirectory(bigint)
-add_subdirectory(cxxopts)
+if (NOT YOSYS_WITH_PKG_DEPS)
+    add_subdirectory(cxxopts)
+endif()
 add_subdirectory(dlfcn-win32)
 add_subdirectory(ezsat)
 add_subdirectory(fst)