Glad support and a few other things

[m00se-3]

The following changes were made to integrate GWork with a side project of mine, hopefully that explains some of the seemingly random changes.

• Added a USE_GLAD option to allow the user to bring in GLAD instead of GLEW for the OPENGL_CORE renderer. GLEW is still the default.
• Added a USE_VCPKG option so that the library can use your VCPKG provided install of GLFW & GLM.
• Bumped the minimum CMAKE version to 3.15, since CMAKE <= 3.5 is going to be unsupported soon.
• Config.cmake checks for the host cpu to be 64-bit rather than checking for Win64. This is because GWork was setting GWK_TARGET_ARCH incorrectly on AMD64 systems.
• Config.cmake checks for mingw rather than for Windows before linking to iconv. Checking for Win32 broke builds on Windows not using MinGW.
• Updated the OpenGLCoreSample program to OpenGL 4.5 because the OpenGLCore renderer uses GL functions only supported in that version. Might consider changing the renderer to use equivalent functions compatible with more OpenGL versions.

In addition to the code reviews, feel free to provide feedback on the pull request overall. This is my first attempt at contributing to open source, so any advice is welcome.