Add a CMake option to select no std::filesystem

Cmake option PEGTL_NO_STD_FILESYSTEM will mean there is
*no* implementation of std::filesystem selected at compile
time. This will invariably lead to a broken PEGTL if you set
it and use file operations, but it is intended to allow a configuration
on systems without boost or std::filesystem, which don't need
filesystem access but do want memory parsing. This allows
taojson memory parsers to compile back to macOS 10.13, for instance.

See discussion in issue #347

CMakeLists.txt

@@ -50,6 +50,7 @@ target_include_directories(pegtl INTERFACE
 target_compile_features(pegtl INTERFACE cxx_std_17)
 
 option(PEGTL_USE_BOOST_FILESYSTEM "Override the auto-detection of std::filesystem and use Boost.Filesystem" OFF)
+option(PEGTL_NO_STD_FILESYSTEM "Don't link to any std::filesystem; file operations will not work" OFF)
 
 # Try compiling a test program with std::filesystem or one of its alternatives
 function(check_filesystem_impl FILESYSTEM_HEADER FILESYSTEM_NAMESPACE OPTIONAL_LIBS OUT_RESULT)
@@ -82,7 +83,10 @@ function(check_filesystem_impl FILESYSTEM_HEADER FILESYSTEM_NAMESPACE OPTIONAL_L
   set(${OUT_RESULT} ${TEST_RESULT} PARENT_SCOPE)
 endfunction()
 
-if(PEGTL_USE_BOOST_FILESYSTEM)
+if (PEGTL_NO_STD_FILESYSTEM)
+  target_compile_definitions(${PROJECT_NAME} INTERFACE TAO_PEGTL_NO_STD_FILESYSTEM)
+  message(STATUS "Skipping std::filesystem in PEGTL; File operations will not work or compile.")
+elseif (PEGTL_USE_BOOST_FILESYSTEM)
   # Force the use of Boost.Filesystem: #include <boost/filesystem.hpp> // boost::filesystem
   find_package(Boost REQUIRED COMPONENTS filesystem)
   target_link_libraries(${PROJECT_NAME} INTERFACE Boost::filesystem)