Hopefully fix <filesystem> check for GCC 7

The CI builds using Ubuntu 18.04 were failing to make it past the CMake
configuration stage due to being unable to find <filesystem> support.
This is because Ubuntu 18.04 ships GCC 7, for which <filesystem> support
was still experimental. As a result, <experimental/filesystem> was
present, not <filesystem>, which causes the CMake check to error out as
the test program #includes <filesystem>.

The fix is simple - use __has_include to include the appropriate header
and add a namespace alias to account for std::experimental.

This is sufficient to get the CMake configuration to succeed using GCC 7
on macOS, but configuration still fails on Ubuntu 18.04 due to the
linker being unable to find the definition for current_path() [0].

This error seems a bit weird since the linker command line clearly
includes -lstdc++fs, and that library definitely defines current_path().
It turns out the Ubuntu 18.04 linker is order-sensitive, and -lstdc++fs
was present on the command line before the object file for the test
program, so the linker doesn't pull in the current_path() definition.
ld64 on macOS, on the other hand, doesn't seem to be sensitive to
library order [1].

Fixing this is fairly simple - use the LINK_LIBRARIES try_compile option
instead of LINK_OPTIONS, so CMake puts the flag in the right place. I
should have done that in the first place, but I somehow glossed over
that option when first writing the new handling.

[0]:
    [ 50%] Linking CXX executable cmTC_6f62e
    /root/cmake-3.27.8-linux-x86_64/bin/cmake -E cmake_link_script CMakeFiles/cmTC_6f62e.dir/link.txt --verbose=1
    /usr/bin/c++  -Wall -Wextra -Wpedantic -Werror  -lstdc++fs CMakeFiles/cmTC_6f62e.dir/std_fs_support_test.cpp.o -o cmTC_6f62e
    CMakeFiles/cmTC_6f62e.dir/std_fs_support_test.cpp.o: In function `main':
    std_fs_support_test.cpp:(.text+0x1f): undefined reference to `std::experimental::filesystem::v1::current_path[abi:cxx11]()'
    collect2: error: ld returned 1 exit status

[1]:
    [100%] Linking CXX executable cmTC_fb65e
    /usr/local/Cellar/cmake/3.27.7/bin/cmake -E cmake_link_script CMakeFiles/cmTC_fb65e.dir/link.txt --verbose=1
    /usr/local/opt/gcc@7/bin/g++-7  -Wall -Wextra -Wpedantic -Werror  -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk -mmacosx-version-min=10.12 -Wl,-search_paths_first -Wl,-headerpad_max_install_names -lstdc++fs CMakeFiles/cmTC_fb65e.dir/std_fs_support_test.cpp.o -o cmTC_fb65e
    ld: warning: dylib (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++.dylib) was built for newer macOS version (10.17.5) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(dir.o)) was built for newer macOS version (10.17) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(ops.o)) was built for newer macOS version (10.17) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(path.o)) was built for newer macOS version (10.17) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(cow-dir.o)) was built for newer macOS version (10.17) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(cow-ops.o)) was built for newer macOS version (10.17) than being linked (10.12)
    ld: warning: object file (/usr/local/Cellar/gcc@7/7.5.0_4/lib/gcc/7/libstdc++fs.a(cow-path.o)) was built for newer macOS version (10.17) than being linked (10.12)
    [100%] Built target cmTC_fb65e

Author: Alex Wang

cmake/std_fs_support_test.cpp

@@ -1,4 +1,11 @@
+#if __has_include(<filesystem>)
 #include <filesystem>
+namespace fs = std::filesystem;
+#else
+#include <experimental/filesystem>
+namespace fs = std::experimental::filesystem;
+#endif
+
 int main() {
-    auto cwd = std::filesystem::current_path();
+    auto cwd = fs::current_path();
 }

include/CMakeLists.txt

@@ -72,7 +72,7 @@ else()
       STDCXXFS_SUPPORTED
       "${CMAKE_CURRENT_BINARY_DIR}/stdcxxfs"
       SOURCES "${PROJECT_SOURCE_DIR}/cmake/std_fs_support_test.cpp"
-      LINK_OPTIONS "-lstdc++fs"
+      LINK_LIBRARIES "stdc++fs"
       CXX_STANDARD 17
       CXX_STANDARD_REQUIRED ON
       CXX_EXTENSIONS OFF
@@ -84,7 +84,7 @@ else()
         CXXFS_SUPPORTED
         "${CMAKE_CURRENT_BINARY_DIR}/cxxfs"
         SOURCES "${PROJECT_SOURCE_DIR}/cmake/std_fs_support_test.cpp"
-        LINK_OPTIONS "-lc++fs"
+        LINK_LIBRARIES "c++fs"
         CXX_STANDARD 17
         CXX_STANDARD_REQUIRED ON
         CXX_EXTENSIONS OFF