Fix std::variant, std::filesystem::path tests on GCC-8, Clang-7,8.

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/std.h

@@ -17,10 +17,17 @@
 #if FMT_HAS_INCLUDE(<version>)
 #  include <version>
 #endif
+// Checking FMT_CPLUSPLUS for warning suppression in MSVC.
+#if FMT_CPLUSPLUS >= 201703L
+#  if FMT_HAS_INCLUDE(<filesystem>)
+#    include <filesystem>
+#  endif
+#  if FMT_HAS_INCLUDE(<variant>)
+#    include <variant>
+#  endif
+#endif
 
 #ifdef __cpp_lib_filesystem
-#  include <filesystem>
-
 FMT_BEGIN_NAMESPACE
 
 namespace detail {
@@ -71,8 +78,6 @@ struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};
 FMT_END_NAMESPACE
 
 #ifdef __cpp_lib_variant
-#  include <variant>
-
 FMT_BEGIN_NAMESPACE
 template <typename Char> struct formatter<std::monostate, Char> {
   template <typename ParseContext>

test/CMakeLists.txt

@@ -78,6 +78,14 @@ add_fmt_test(printf-test)
 add_fmt_test(ranges-test ranges-odr-test.cc)
 add_fmt_test(scan-test)
 add_fmt_test(std-test)
+try_compile(compile_result_unused
+            ${CMAKE_CURRENT_BINARY_DIR}
+            SOURCES ${CMAKE_CURRENT_LIST_DIR}/detect-stdfs.cc
+            OUTPUT_VARIABLE RAWOUTPUT)
+string(REGEX REPLACE ".*libfound \"([^\"]*)\".*" "\\1" STDLIBFS "${RAWOUTPUT}")
+if (STDLIBFS)
+  target_link_libraries(std-test ${STDLIBFS})
+endif ()
 add_fmt_test(unicode-test HEADER_ONLY)
 if (MSVC)
   target_compile_options(unicode-test PRIVATE /utf-8)

test/detect-stdfs.cc

+// Formatting library for C++ - tests of formatters for standard library types
+//
+// Copyright (c) 2012 - present, Victor Zverovich
+// All rights reserved.
+//
+// For the license information refer to format.h.
+
+#include <exception>  // _GLIBCXX_RELEASE & _LIBCPP_VERSION
+
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 8
+#  error libfound "stdc++fs"
+#elif !defined(__apple_build_version__) && defined(_LIBCPP_VERSION) && \
+    _LIBCPP_VERSION >= 7000 && _LIBCPP_VERSION < 9000
+#  error libfound "c++fs"
+#else
+// none if std::filesystem does not require additional libraries
+#  error libfound ""
+#endif