nc_cxx: make stdc++fs discovery more robust

nc_cxx.cmake

@@ -8,10 +8,21 @@ append_compile_flag (-std=c++1z)
 
 ###############################################################################
 # find the gcc experimental filesystem library and append to libs if needed
-find_library(STDCXXFS NAMES stdc++fs)
-if (STDCXXFS)
-    set(LIBS ${LIBS} ${STDCXXFS})
-endif (STDCXXFS)
+#
+# note, we specifically _cannot_ use find_library because it just makes wild
+# hardcoded guess and doesn't respect _all_ the system paths; specifically
+# something like /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/libstdc++.so won't get
+# discovered.
+foreach(lib "" stdc++fs)
+    if (NOT __nc_cxx_stdcxxfs)
+        try_compile(__nc_cxx_stdcxxfs ${CMAKE_CURRENT_BINARY_DIR}
+            SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/cmake/nc_cxx_stdcxxfs.cpp
+            LINK_LIBRARIES ${lib})
+        if (__nc_cxx_stdcxxfs)
+            list (APPEND LIBS ${lib})
+        endif ()
+    endif ()
+endforeach()
 
 
 ###############################################################################

nc_cxx_stdcxxfs.cpp

@@ -0,0 +1,11 @@
+#include <experimental/filesystem>
+
+int
+main (int argc, char **argv)
+{
+    (void)argc;
+    (void)argv;
+
+    std::experimental::filesystem::path p { "foo" };
+    return 0;
+}