Fix use of std::filesystem on older GCC and Clang

[phongn]

Older versions of GCC (< 9.1) and Clang (< 9) required use of additional library linking flags in order to use std::filesystem. This PR instructs cmake to use those libstdc++/libc++ filesystem libraries when appropriate.

[cmcfarlen]

I don't have an older compiler to check with, but if this works for you and doesn't break the build then Im 👍

[ezelkow1]

This didnt seem to fix it for me (on gcc 8.5)

[ 52%] Building CXX object src/iocore/cache/CMakeFiles/Cache.dir/unit_tests/test_Cache.cc.o
[ 52%] Linking CXX executable Cache
libinkcache.a(Cache.cc.o): In function persist_bad_disks()': Cache.cc:(.text+0x56c1): undefined reference to std::filesystem::__cxx11::path::parent_path() const'
Cache.cc:(.text+0x56d7): undefined reference to std::filesystem::create_directories(std::filesystem::__cxx11::path const&, std::error_code&)' libinkcache.a(Cache.cc.o): In function std::filesystem::__cxx11::path::operator/=(std::filesystem::__cxx11::path const&)':
Cache.cc:(.text.ZNSt10filesystem7__cxx114pathdVERKS1[ZNSt10filesystem7__cxx114pathdVERKS1]+0x3d): undefined reference to std::filesystem::__cxx11::path::has_filename() const' Cache.cc:(.text._ZNSt10filesystem7__cxx114pathdVERKS1_[_ZNSt10filesystem7__cxx114pathdVERKS1_]+0x98): undefined reference to std::filesystem::__cxx11::path::_M_split_cmpts()'
libinkcache.a(Cache.cc.o): In function std::filesystem::__cxx11::path::is_absolute() const': Cache.cc:(.text._ZNKSt10filesystem7__cxx114path11is_absoluteEv[_ZNKSt10filesystem7__cxx114path11is_absoluteEv]+0x14): undefined reference to std::filesystem::__cxx11::path::has_root_directory() const'
libinkcache.a(Cache.cc.o): In function std::filesystem::__cxx11::path::path<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::filesystem::__cxx11::path>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::filesystem::__cxx11::path::format)': Cache.cc:(.text._ZNSt10filesystem7__cxx114pathC2INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE[_ZNSt10filesystem7__cxx114pathC5INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE]+0x64): undefined reference to std::filesystem::__cxx11::path::_M_split_cmpts()'
libinkcache.a(Cache.cc.o): In function std::filesystem::__cxx11::path::path<char const*, std::filesystem::__cxx11::path>(char const* const&, std::filesystem::__cxx11::path::format)': Cache.cc:(.text._ZNSt10filesystem7__cxx114pathC2IPKcS1_EERKT_NS1_6formatE[_ZNSt10filesystem7__cxx114pathC5IPKcS1_EERKT_NS1_6formatE]+0x64): undefined reference to std::filesystem::__cxx11::path::_M_split_cmpts()'
collect2: error: ld returned 1 exit status
gmake[2]: *** [src/iocore/cache/CMakeFiles/Cache.dir/build.make:185: src/iocore/cache/Cache] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:4225: src/iocore/cache/CMakeFiles/Cache.dir/all] Error 2
gmake: *** [Makefile:146: all] Error 2

just to double check I do see new cmake changes in my branch

Older LLVM and GCC compilers with C++17 support require explicit linking to their std::filesystem

libraries

if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.1)
target_link_libraries(traffic_server PRIVATE stdc++fs)
endif()

if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9)
target_link_libraries(traffic_server PRIVATE c++fs)
endif()

[phongn]

@ezelkow1 resolved in latest update to this PR, though we have a larger conversation on if we want to allow this.

[ezelkow1]

Yes this does fix the issue in my testing now. However we do need that conversation if we want to allow the older compilers or not. If so then Im good with it

[bryancall]

@phongn We are not officially supporting GCC compilers less than GCC 10 for the ATS 10 release. Do you need this PR or can we close it?