Ehh dont use the std::filesystem implementation by default

Author: franzpoeschel

.github/workflows/windows.yml

@@ -66,11 +66,12 @@ jobs:
 
         call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\vc\Auxiliary\build\vcvarsall.bat" x64
         pwsh "share\openPMD\download_samples.ps1" build
-        cmake -S . -B build   ^
-              -G "Ninja"      ^
-              -DCMAKE_C_COMPILER=clang-cl   ^
-              -DCMAKE_CXX_COMPILER=clang-cl ^
-              -DCMAKE_BUILD_TYPE=Release    ^
+        cmake -S . -B build                       ^
+              -G "Ninja"                          ^
+              -DCMAKE_C_COMPILER=clang-cl         ^
+              -DCMAKE_CXX_COMPILER=clang-cl       ^
+              -DCMAKE_BUILD_TYPE=Release          ^
+              -DopenPMD_USE_FILESYSTEM_HEADER=OFF ^
               -DopenPMD_USE_MPI=OFF
         cmake --build build --config Release --parallel 2
         cmake --build build --config Debug --target install

CMakeLists.txt

@@ -151,7 +151,7 @@ option(openPMD_USE_INTERNAL_TOML11   "Use internally shipped toml11"        ${op
 
 option(openPMD_USE_INVASIVE_TESTS "Enable unit tests that modify source code" OFF)
 option(openPMD_USE_VERIFY "Enable internal VERIFY (assert) macro independent of build type" ON)
-option(openPMD_USE_FILESYSTEM_HEADER "Enable filesystem header. May be disabled for old compiler versions." ON)
+option(openPMD_USE_FILESYSTEM_HEADER "Enable filesystem header. May be disabled for old compiler versions." OFF)
 mark_as_advanced(openPMD_USE_FILESYSTEM_HEADER)
 
 set(CMAKE_CONFIGURATION_TYPES "Release;Debug;MinSizeRel;RelWithDebInfo")