cmake: extract std::filesystem linkage checking into module

see
https://libcxx.llvm.org/docs/UsingLibcxx.html#using-libc-experimental-and-experimental,

> Note that as of libc++ 7.0 using the <experimental/filesystem>
> requires linking libc++fs instead of libc++experimental.

do not build ceph_test_admin_socket_output if we are not able to find
the library for std::experimental::filesystem . it is a workaround of
https://reviews.freebsd.org/D10840 where FreeBSD 11.2 does not ship
libc++experimental.a .

Signed-off-by: Kefu Chai <kchai@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/cmake/modules/FindStdFilesystem_test.cc b/cmake/modules/FindStdFilesystem_test.cc
new file mode 100644
index 00000000000..82f2a8b576b
--- /dev/null
+++ b/cmake/modules/FindStdFilesystem_test.cc
@@ -0,0 +1,13 @@
+#if __has_include(<filesystem>)
+#include <filesystem>
+namespace fs = std::filesystem;
+#elif __has_include(<experimental/filesystem>)
+#include <experimental/filesystem>
+namespace fs = std::experimental::filesystem;
+#else
+#error std::filesystem not available!
+#endif
+
+int main() {
+  [[maybe_unused]] fs::path path("/");
+}