* Improve cmake config file generation

This allows for finding raylib on a non-standard location (eg, not in /usr/lib{,64})

* Only have glfw as private if using internal glfw and not static

Closes #3265.

The problem: LIBS_PRIVATE is a list of library names (used by pkg-config), but the shared library of the same name doesn't always exist.

As described in the official documentation https://cmake.org/cmake/help/v3.0/variable/BUILD_SHARED_LIBS.html this flag is global by default and controls if the library will be built as a shared or a static library allowing us to define only one call to the add_library function (without specifying its type). It is also added as an option to be visible in CMake GUI applications.

* Delete emscripten.cmake

This file is not needed at this point. EMSDK provides a toolchain file that has a lot more things in it and is better supported. Project currently works fine with the documentation provided in Emscripten SDK on how to build projects.

* First pass file separation.

The main two files are cleaner now. Only important things can be seen. Major changes include:
- raylib_static is now the alias instead of raylib
- Repeating segments are removed and pulled into separate files into <root>/cmake
- File is reordered to make more sense
- Installs are better structured
- Library is build into an output directory "raylib" instead of "src"
- All public header files are now set as a public header file
- Source files need to be listed (it is a bad practice to capture them using wildcards and file globs)
- CMakeLists are better commented

* Second pass on the example dirs.

They are quite complex so I'm more hesitant to do major changes. Also it works pretty well. Noticed that I forgot one of the seperated files and added it into src/CMakeLists.txt.

* Returned the header copy as it was convenient to have the public headers copied.

* A better description to the variable RAYLIB_IS_MAIN

Co-authored-by: Rob Loach <robloach@gmail.com>

* Remove debug message

Co-authored-by: Rob Loach <robloach@gmail.com>

* Improvements based on review.

* Simplify the install condition to not be platform specific as it was before.

Co-authored-by: Alexander Neumann <30894796+Neumann-A@users.noreply.github.com>

* Remove some CMAKE variables as they don't affect the build in any way

Co-authored-by: Alexander Neumann <30894796+Neumann-A@users.noreply.github.com>

Co-authored-by: Rob Loach <robloach@gmail.com>
Co-authored-by: Alexander Neumann <30894796+Neumann-A@users.noreply.github.com>