cmake: auto-detect if std::filesystem needs -lstdc++fs (fixes RHEL8/gcc-8.5)

On gcc-8 (e.g. RHEL8's system compiler, 8.5), std::filesystem is present
but still split out into a separate archive, libstdc++fs.a, so linking
msr2msr failed with undefined references such as
std::filesystem::remove(std::filesystem::__cxx11::path const&, std::error_code&).
gcc>=9 and clang/libc++ ship it in the main runtime library already.

Detect this via an actual compile+link try (first with no extra lib, then
with -lstdc++fs/-lc++fs) instead of guessing from compiler/OS, so
macOS, RHEL9/10, Ubuntu, etc. are unaffected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -162,6 +162,45 @@ find_package(FFTW3 REQUIRED)
 #--- check for libxml2 --------------------------------------------------------
 find_package(LibXml2 REQUIRED)
 
+#--- check whether <filesystem> needs an extra library to link ----------------
+#--- On most current toolchains (gcc>=9, clang/libc++, MSVC, macOS) std::filesystem
+#--- lives in the main runtime library and just works. On gcc-8 (e.g. RHEL8's
+#--- system gcc, 8.5) it is present but still split out into a separate static
+#--- archive, libstdc++fs.a, and needs an explicit -lstdc++fs, otherwise linking
+#--- fails with 'undefined reference to std::filesystem::remove(...)' etc.
+#--- Detected via an actual compile+link try, so this adapts automatically
+#--- instead of guessing from compiler/OS name and risks affecting unrelated
+#--- platforms. -----------------------------------------------------------
+include(CheckCXXSourceCompiles)
+set(STDFS_TEST_SRC "
+#include <filesystem>
+int main() {
+  std::filesystem::path p(\"foo\");
+  std::error_code ec;
+  std::filesystem::remove(p, ec);
+  return 0;
+}
+")
+check_cxx_source_compiles("${STDFS_TEST_SRC}" HAVE_STD_FILESYSTEM_NOLIB)
+
+set(STDFS_LIBRARY "")
+if (NOT HAVE_STD_FILESYSTEM_NOLIB)
+  foreach (_stdfs_lib stdc++fs c++fs)
+    set(CMAKE_REQUIRED_LIBRARIES ${_stdfs_lib})
+    unset(HAVE_STD_FILESYSTEM_WITH_${_stdfs_lib} CACHE)
+    check_cxx_source_compiles("${STDFS_TEST_SRC}" HAVE_STD_FILESYSTEM_WITH_${_stdfs_lib})
+    unset(CMAKE_REQUIRED_LIBRARIES)
+    if (HAVE_STD_FILESYSTEM_WITH_${_stdfs_lib})
+      set(STDFS_LIBRARY ${_stdfs_lib})
+      break ()
+    endif ()
+  endforeach ()
+  if (NOT STDFS_LIBRARY)
+    message(FATAL_ERROR "std::filesystem is required but no linkable implementation could be found")
+  endif ()
+  message(STATUS "std::filesystem needs extra library: ${STDFS_LIBRARY}")
+endif ()
+
 #--- check for OpenMP ---------------------------------------------------------
 if (try_OpenMP)
   #--- AppleClang has no built-in OpenMP runtime; MacPorts' 'libomp' port

src/CMakeLists.txt

@@ -76,7 +76,7 @@ target_include_directories(msr2msr
   BEFORE PRIVATE 
     $<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/src/include>
 )
-target_link_libraries(msr2msr ${ROOT_LIBRARIES})
+target_link_libraries(msr2msr ${ROOT_LIBRARIES} ${STDFS_LIBRARY})
 
 add_executable(musrfit musrfit.cpp)
 target_compile_options(musrfit BEFORE PRIVATE "-DHAVE_CONFIG_H" "${HAVE_GIT_REV_H}")