Add support for Apple framework builds

[Treata11]

[Treata11]

Greetings,

In I introduced the required properties for framework generation as described in CMake documentation

I built the project with the following:

cmake -S. -B build_iOS \
-DBUILD_SHARED_LIBS=OFF \
-DBUILD_CODEC=OFF \
-DOPJ_FRAMEWORK=ON \
-DCMAKE_SYSTEM_NAME=iOS

sudo cmake --build build_iOS --target install --config Release

And the framework artifact is created at build_iOS/bin/openjp2.framework with openjpeg.h & opj_config.h as the public headers & also some resources.

[rouault]

I've tried this PR on Linux and "make install" fails with:

CMake Error at cmake_install.cmake:80 (file):
  file INSTALL cannot find
  "/home/even/openjpeg/openjpeg/build/OpenJPEGTargets.cmake": No such file or
  directory.