Describe the bug
Enabling CMAKE_INTERPROCEDURAL_OPTIMIZATION in 93e77ba breaks my cross builds from linux to windows.

Some messages from the build

...
x86_64-w64-mingw32/bin/ar: deps/libmd5/CMakeFiles/md5.dir/md5.c.obj: plugin needed to handle lto object
x86_64-w64-mingw32/bin/ranlib: lib/libmd5.a(md5.c.obj): plugin needed to handle lto object
...
x86_64-w64-mingw32/bin/ld: src/CMakeFiles/doxygen.dir/main.cpp.obj: plugin needed to handle lto object
x86_64-w64-mingw32/bin/ld: lib/libmingw32.a(lib64_libmingw32_a-crtexewin.o): in function `main':
mingw-w64-crt/crt/crtexewin.c:66:(.text.startup+0xb4): undefined reference to `WinMain'

It may be a toolchain issue on my side.

To Reproduce
You will need a x86_64 mingw toolchain with some dependencies. So no easy one-liner to reproduce.

Expected behavior
Just build without any issues.

Version
1.15.0

Additional context
Simplest fix would be to make CMAKE_INTERPROCEDURAL_OPTIMIZATION an option (default TRUE) so that it can be set from the outside if causing issues.