[DRAFT, WIP] Detect presence of flockfile

Just a dry-run at a possible implementation pending clarification on
details.

Fixes #4646

Author: mvastola

CMakeLists.txt

@@ -128,6 +128,7 @@ endif ()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/support/cmake")
 
 include(CheckCXXCompilerFlag)
+include(CheckCXXSourceCompiles)
 include(JoinPaths)
 
 if (FMT_MASTER_PROJECT AND NOT DEFINED CMAKE_CXX_VISIBILITY_PRESET)
@@ -307,6 +308,18 @@ else ()
   message(WARNING "Feature cxx_std_11 is unknown for the CXX compiler")
 endif ()
 
+check_cxx_source_compiles([[
+    #include <stdio.h>
+    int main() {
+        // Use stdout just to prove we can pass a FILE* to it
+        flockfile(stdout);
+        funlockfile(stdout);
+        return 0;
+    }
+]] HAVE_FLOCKFILE_COMPILE_TEST)
+target_compile_definitions(fmt PRIVATE
+  FMT_HAVE_FLOCKFILE_COMPILE_TEST=$<BOOL:HAVE_FLOCKFILE_COMPILE_TEST>)
+
 # Set FMT_LIB_NAME for pkg-config fmt.pc. We cannot use the OUTPUT_NAME target
 # property because it's not set by default.
 set(FMT_LIB_NAME fmt)

include/fmt/format-inl.h

@@ -1490,7 +1490,11 @@ template <typename F> auto getc_unlocked(F* f) -> decltype(_fgetc_nolock(f)) {
 #endif
 
 #ifndef FMT_USE_FLOCKFILE
-#  define FMT_USE_FLOCKFILE 1
+#  ifdef FMT_HAVE_FLOCKFILE_COMPILE_TEST
+#    define FMT_USE_FLOCKFILE FMT_HAVE_FLOCKFILE_COMPILE_TEST
+#  else
+#    define FMT_USE_FLOCKFILE 1
+#  endif
 #endif
 
 template <typename F = FILE, typename Enable = void>