Modernize the CMake build system

[randomusername-a]

• Bump the minimum CMake version to 3.5
• Consistently use tabs instead of four spaces
• Don't ship our own FindSDL2*.cmake files. SDL2 ships their own in modern versions.
• Don't unnecessarily check if a dependency was found after setting that dependency as REQUIRED.
• Prefer to use GLVND by setting the OpenGL_GL_PREFERENCE variable before calling find_package for OpenGL
• Use the more modern targets (eg. OpenGL::GL) instead of variables to link to libraries and set include dirs.
• Don't hardcode /usr/local/bin as the binary dir, instead use the user-configurable ${CMAKE_INSTALL_BINDIR} variable

[shonumi]

Looks like some good changes. I'll have to test it on some of my machines before merging. Might be a day or two until then.

One thing however, is that GBE+ uses tabs instead of 4 spaces. I dunno if its standard for other projects to use 4 spaces in CMake files, but I'm pretty firm about my preference for tabs.

[randomusername-a]

There were mixed spaces and tabs before my changes. I've replaced them all with tabs now.

[shonumi]

Thanks! I believe those spaces were from outside sources (old copy+pastes and one other contributor), so sometimes they sneak in.

Like I said, I'll test it on my end (Linux and Windows) and merge once it checks out.

[randomusername-a]

Sure, no problem.