cmake: install config-module
Installs a config module if CMake verion >= 2.8.12. The config-module creates the import library targets built in the project (glew, glew_s, glewmx, glewmx_s) but in accordance with the FindGLEW module shipped with CMake, it also creates GLEW::GLEW and GLEW::GLEWMX. GLEW::GLEW and GLEW::GLEWMX will be simply copies of glew/glewmx or glew_s/glewmx_s. If both versions are available they alias the shared versions. The default behaviour can be changed either when installing or when using the package: - Set BUILD_SHARED_LIBS to OFF or ON when building and installing GLEW. This controls which libraries (shared or static) will be installed (and not which will be built). - Set GLEW_USE_STATIC_LIBS to OFF or ON before calling `find_package(GLEW CONFIG REQUIRED)` to force the config-module to create GLEW::GLEW and GLEWMX as aliases to glew/glewmx or glew_s/glewmx_s The script ./cmake-testbuild.sh is added to test the CMake build and config-module. See instructions there.