CMake: Allow install if non-toplevel project.

[KerstinKeller]

If another project vendors termcolor (via add_subdirectory) and tries to install, the resulting package is not usable.
Let's say the top level project has a CMakeLists.txt like this:

cmake_minimum_required(VERSION 3.20)

project(my_project)
add_subdirectory(termcolor)

The generated termcolor-config.cmake will want to include my_project-targets.cmake instead of termcolor-targets.cmake.

By changing CMAKE_PROJECT_NAME to PROJECT_NAME in config.cmake.in this problem is solved.