build: Avoid rebuilds due to processing of fmt headers (#4313)

This continues the adventure from #4082 where the processing of fmt
headers, both locally built and externally referenced, causes large
rebuilds to occur when making small or even zero changes between builds.

Since that previous PR is stuck I'll try again here.

This makes two sets of changes:
- Check if our generated files exist before writing to them (in the
externally referenced case)
- Use `copy_if_different` to prevent redundant copies from occuring, and
triggering timestamp updates (in the local build case)

Note that while this seems to fix the problem, the build is still not
"clean" wrt fmt processing. The following will be be seen when
triggering a build, with zero changes since the last build, within VS:
```
1>------ Build started: Project: ZERO_CHECK, Configuration: Debug x64 ------
1>1>Checking Build System
2>------ Build started: Project: fmt_internal_target, Configuration: Debug x64 ------
3>------ Build started: Project: CopyFiles, Configuration: Debug x64 ------

2>Generating ../../include/OpenImageIO/detail/fmt
2>Building Custom Rule C:/Users/jesse/source/oiio/src/include/CMakeLists.txt
2>Generating ../../include/OpenImageIO/detail/fmt/core.h, ../../include/OpenImageIO/detail/fmt/format-inl.h, ../../include/OpenImageIO/detail/fmt/format.h, ../../include/OpenImageIO/detail/fmt/ostream.h, ../../include/OpenImageIO/detail/fmt/printf.h, ../../include/OpenImageIO/detail/fmt/std.h, ../../include/OpenImageIO/detail/fmt/chrono.h
2>C:\Program Files\Microsoft Visual Studio\2022\Preview\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): warning MSB8065: Custom build for item "C:\Users\jesse\source\oiio\build\CMakeFiles\b19a1b09c2e7eaa53ca90801840dfdd0\fmt.rule" succeeded, but specified output "c:\users\jesse\source\oiio\build\include\openimageio\detail\fmt" has not been created. This may cause incremental build to work incorrectly.
2>C:\Program Files\Microsoft Visual Studio\2022\Preview\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): warning MSB8064: Custom build for item "C:\Users\jesse\source\oiio\build\CMakeFiles\b253238471e54ff57d14ae397110f40e\core.h.rule" succeeded, but specified dependency "c:\users\jesse\source\oiio\build\include\openimageio\detail\fmt" does not exist. This may cause incremental build to work incorrectly.

2>Done building project "fmt_internal_target.vcxproj".
4>------ Build started: Project: OpenImageIO_Util, Configuration: Debug x64 ------
```
Whatever it is, the VS generator doesn't much like the way we're
handling the processing but I don't know enough cmake-fu to ascertain
why that might be.


Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

src/include/CMakeLists.txt

@@ -86,7 +86,7 @@ if (OIIO_INTERNALIZE_FMT OR fmt_LOCAL_BUILD)
     add_custom_command (OUTPUT ${fmt_headers_internal}
                         DEPENDS ${fmt_headers} ${fmt_internal_directory}
                         COMMAND
-                            ${CMAKE_COMMAND} -E copy
+                            ${CMAKE_COMMAND} -E copy_if_different
                             ${fmt_headers}
                             ${fmt_internal_directory})
     add_custom_target (fmt_internal_target DEPENDS ${fmt_headers_internal})
@@ -96,13 +96,17 @@ else ()
          ${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/ostream.h
          ${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/printf.h )
     foreach (f format.h ostream.h printf.h)
-        file (WRITE "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/${f}"
-                    "#include <fmt/${f}>")
+        if (NOT EXISTS "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/${f}")
+            file (WRITE "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/${f}"
+                        "#include <fmt/${f}>")
+        endif ()
     endforeach ()
     if (fmt_VERSION VERSION_GREATER_EQUAL 9)
         list (APPEND fmt_headers ${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/std.h)
-        file (WRITE "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/std.h"
-                    "#include <fmt/std.h>")
+        if (NOT EXISTS "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/std.h")
+            file (WRITE "${CMAKE_BINARY_DIR}/include/OpenImageIO/detail/fmt/std.h"
+                        "#include <fmt/std.h>")
+        endif ()
     endif ()
 endif ()
 install (FILES ${fmt_headers}