Switch to C++17 for Mac OS X

Makefile.homebrew

@@ -5,7 +5,7 @@ SSLROOT = ${BREWROOT}/opt/openssl@1.1
 UPNPROOT = ${BREWROOT}/opt/miniupnpc
 
 CXXFLAGS ?= ${CXX_DEBUG} -Wall -Wno-overloaded-virtual
-NEEDED_CXXFLAGS ?= -std=c++11
+NEEDED_CXXFLAGS ?= -std=c++17
 INCFLAGS ?= -I${SSLROOT}/include -I${BOOSTROOT}/include
 LDFLAGS ?= ${LD_DEBUG}
 DEFINES += -DMAC_OSX

Makefile.osx

@@ -1,5 +1,5 @@
 CXX = clang++
-CXXFLAGS := ${CXX_DEBUG} -Wall -std=c++11
+CXXFLAGS := ${CXX_DEBUG} -Wall -std=c++17
 INCFLAGS = -I/usr/local/include
 DEFINES := -DMAC_OSX
 LDFLAGS := -Wl,-rpath,/usr/local/lib -L/usr/local/lib

libi2pd/FS.cpp

@@ -9,7 +9,9 @@
 #include <algorithm>
 
 #if defined(MAC_OSX)
+#if !STD_FILESYSTEM
 #include <boost/system/system_error.hpp>
+#endif
 #include <TargetConditionals.h>
 #endif
 

libi2pd/FS.h

@@ -16,7 +16,7 @@
 #include <functional>
 
 #ifndef STD_FILESYSTEM
-#	if (!defined(MAC_OSX) && !TARGET_OS_SIMULATOR && \
+#	if (!TARGET_OS_SIMULATOR && \
 	(__cplusplus >= 201703L) && __has_include(<filesystem>)) // C++ 17 or higher and supports std::filesystem
 #		define STD_FILESYSTEM 1
 #	else