Don't include fmt/os.h in the module if it is disabled via FMT_OS

CMakeLists.txt

@@ -150,7 +150,7 @@ option(FMT_INSTALL "Generate the install target." ON)
 option(FMT_TEST "Generate the test target." ${FMT_MASTER_PROJECT})
 option(FMT_FUZZ "Generate the fuzz target." OFF)
 option(FMT_CUDA_TEST "Generate the cuda-test target." OFF)
-option(FMT_OS "Include core requiring OS (Windows/Posix) " ON)
+option(FMT_OS "Include OS-specific APIs." ON)
 option(FMT_MODULE "Build a module instead of a traditional library." OFF)
 option(FMT_SYSTEM_HEADERS "Expose headers with marking them as system." OFF)
 
@@ -280,6 +280,7 @@ add_headers(FMT_HEADERS args.h chrono.h color.h compile.h core.h format.h
 set(FMT_SOURCES src/format.cc)
 if (FMT_OS)
   set(FMT_SOURCES ${FMT_SOURCES} src/os.cc)
+  target_compile_definitions(fmt PRIVATE FMT_OS=0)
 endif ()
 
 add_module_library(fmt src/fmt.cc FALLBACK

src/fmt.cc

@@ -90,7 +90,9 @@ extern "C++" {
 #include "fmt/color.h"
 #include "fmt/compile.h"
 #include "fmt/format.h"
-#include "fmt/os.h"
+#if !defined(FMT_OS) || FMT_OS
+# include "fmt/os.h"
+#endif
 #include "fmt/printf.h"
 #include "fmt/std.h"
 #include "fmt/xchar.h"