build(win): only pass /MANIFEST:NO to MSVC (#5129)

Fixes #5124

`fancy_add_executable()` currently adds the Windows manifest resource
file for all
`WIN32` builds, but it also unconditionally passes `/MANIFEST:NO` to the
linker.
That switch is MSVC-specific, so MinGW ends up forwarding it to `ld` as
a path
and the executable link fails.

This change keeps embedding `src/windows/oiio_exe.rc` for all Windows
toolchains,
but only adds `/MANIFEST:NO` when the build is using MSVC.

That preserves the intended long-path manifest behavior from #5066 while
letting
MinGW link the Windows executables again.

Assisted-by: Codex / GPT-5

Tests:

- Ran a targeted CMake smoke test that configures a dummy executable
with
  `WIN32=TRUE,MSVC=TRUE` and verified that `/MANIFEST:NO` is present in
  `LINK_OPTIONS` and the `.rc` file is attached as a source.
- Ran the same smoke test with `WIN32=TRUE,MSVC=FALSE` and verified that
the
  `.rc` file is still attached but `/MANIFEST:NO` is not added.
- I could not run an actual MinGW build locally in this environment
because a
  MinGW toolchain is not installed here.

Signed-off-by: Asish Kumar <officialasishkumar@gmail.com>

Author: officialasishkumar

src/cmake/fancy_add_executable.cmake

@@ -45,8 +45,10 @@ macro (fancy_add_executable)
     if (_target_NAME_enabled)
         add_executable (${_target_NAME} ${_target_SRC})
         if (WIN32)
-            # Disable default manifest generation to avoid conflicts.
-            target_link_options(${_target_NAME} PRIVATE "/MANIFEST:NO")
+            if (MSVC)
+                # Disable default manifest generation to avoid conflicts.
+                target_link_options(${_target_NAME} PRIVATE "/MANIFEST:NO")
+            endif ()
             # Include Windows resource file, which will in turn embed our exe manifest.
             target_sources(${_target_NAME} PRIVATE "${PROJECT_SOURCE_DIR}/src/windows/oiio_exe.rc")
         endif ()