FindGLEW: Update implementation

With this PR, I updated the FindGLEW module to replicate upstream behaviour and, in particular, to have the following features:

• define the imported target GLEW::glew for the shared library GLEW;
• if GLEW_USE_STATIC_LIBS is defined and set to TRUE, this module instad defines the imported target GLEW::glew_s for the static library GLEW;
• an imported target GLEW::GLEW is created as a copy of either GLEW::glew or GLEW::glew_s;
• by setting GLEW_VERBOSE you print out the log of the module.

Fixes: #18989 (closed)
Fixes: #17864 (closed)
Fixes: #17638 (closed)
Topic-rename: FindGLEW-updates