Converting to C++
In fact, not only you should correct the linker script specification (use a relative path, like ..\STM32F746NGHx_FLASH.ld rather than an absolute one), but you also must add linker flags -specs=nosys.specs -specs=nano.specs in C/C++ Build >> Settings >> MCU G++ Linker >> Miscellaneous
Then the project should compile correctly.
These are glitches in C to C++ conversion that will be corrected in a future release.
Bernard