replace boost filesystem with std filesystem; upgrade gtest and find it using cmake.

webcc/logger.h

@@ -9,7 +9,12 @@
 #include <cstring>  // for strrchr()
 #include <string>
 
-#include "boost/filesystem/path.hpp"
+// Avoid include <filesystem> in the header.
+namespace std {
+namespace filesystem {
+class path;
+}  // namespace filesystem
+}  // namespace std
 
 // Log levels.
 // VERB is similar to DEBUG commonly used by other projects.
@@ -44,7 +49,7 @@ const int LOG_FILE_OVERWRITE = LOG_FILE | LOG_OVERWRITE;
 
 // Initialize logger.
 // If |dir| is empty, log file will be generated in current directory.
-void LogInit(const boost::filesystem::path& dir, int modes);
+void LogInit(const std::filesystem::path& dir, int modes);
 
 void Log(int level, const char* file, int line, const char* format, ...);