[BUG] Build issue with libraw

[OgreTransporter]

In PR #4556 the option COMPILE_OPTIONS was added to the macro add_oiio_plugin which adds these to the plugin with target_compile_options. The reason for this was to pass the variable LibRaw_DEFINITIONS to the compiler. Unfortunately a mistake was made, because in FindLibRaw.cmake lines 99 and 100 this variable is used as a list for definitions as the name suggests.

OpenImageIO/src/cmake/modules/FindLibRaw.cmake

Line 99 in e964a6a

set (LibRaw_r_DEFINITIONS ${LibRaw_r_DEFINITIONS} LIBRAW_NODLL)

The original code in the CMakeLists.txt of the raw plugin DEFINITIONS "USE_LIBRAW=1" ${LibRaw_r_DEFINITIONS} was therefore correct. The new variant COMPILE_OPTIONS ${LibRaw_r_DEFINITIONS} DEFINITIONS "USE_LIBRAW=1" treats the list of definitions as compiler flags, which does not work. LIBRAW_NODLL is not a valid compiler argument, so an attempt is made to compile the file LIBRAW_NODLL. As this file does not exist, an error occurs.

It is easy to fix by adding -D to lines 99 and 100 of FindLibRaw.cmake:

        set (LibRaw_r_DEFINITIONS ${LibRaw_r_DEFINITIONS} -D LIBRAW_NODLL)
        set (LibRaw_DEFINITIONS ${LibRaw_DEFINITIONS} -D LIBRAW_NODLL)

[lgritz]

I fear that COMPILER_OPTIONS won't work on Windows. When put in DEFINITIONS, cmake knows to convert any -D's to /D, but I'm not sure it will do that for COMPILER_OPTIONS.

So I think what we want is the combination of adding those -D back in, but also partially revering that other PR to move this whole thing back to DEFINITIONS rather than OPTIONS.

[OgreTransporter]

Here I think there is a misunderstanding of the functions target_compile_options and target_compile_definitions. target_compile_options adds options to the compiler without any changes. The user is responsible for ensuring that these are correct. If you want to set a definition, you must therefore also specify -D. target_compile_definitions treats all passed values as definitions and automatically appends them to the compiler with -D. This is the normal behaviour as intended by CMake.