fix(cmake): fix installation with cmake --install

[etag4048]

fixes issue #6787 #6478

The DESTINATION arg of the install directives located in env_support/cmake/custom.cmake being hardcoded to the default value of CMAKE_INSTALL_PREFIX resulted in a broken cmake_install.cmake file

If the user wants to use cmake to install lvgl headers and libraries to a custom location it can be done
at installation time with the following command cmake --install build_dir --prefix /usr/custom/

Please note that currently lvgl.pc is not generated properly when --prefix is used.
There is an entire discussion on this subject in the cmake forums, the workaround is to set CMAKE_INSTALL_PREFIX at configuration time using the -D argument

https://discourse.cmake.org/t/how-to-generate-pc-pkg-config-file-supporting-prefix-of-the-cmake-install/4109

[kisvegabor]

#7127 is merged. Is it ready to merge too?

[etag4048]

@kisvegabor I will attempt to add the missing commit before the Thursday evening, now busy with other things.

[kisvegabor]

Let us know when it's ready to review.

[etag4048]

@kisvegabor Ready.

[kisvegabor]

It's hard to tell for me if it's really better this way, but I trust your expertise 🙂

[etag4048]

@kisvegabor It works with or without specifying a prefix, I've also removed the paths that are destined for executables. Since lvgl is a library, not an executable.