This both reduces redundancy in add_executable definitions, and makes it easier to link additional libraries. In particular, extra libraries are needed on OSX - see next commit.

Hopefully this will make people stop thinking it's a hard dependency.

While not having a noticeable effect on CPU-bound applications, this
change gives an about 30-50% increase in performance for games using
the GPU.

The X11 libraries don't need to be specified when doing dynamic linking

Uses QSettings on citra-qt, and inih on citra-cli.

It only causes issues when someone (who doesn't need it) doesn't have it.

Several cleanups to the buildsystem:
 - Do better factoring of common libs between platforms.
 - Add support to building on Windows.
 - Remove Qt4 support.
 - Re-sort file lists and add missing headers.

This should fix the GL loading errors that occur in some drivers due to
the use of deprecated functions by GLEW. Side benefits are more accurate
auto-completion (deprecated function and symbols don't exist) and faster
pointer loading (less entrypoints to load). In addition it removes an
external library depency, simplifying the build system a bit and
eliminating one set of binary libraries for Windows.

After adding FindGLEW.cmake to externals, the variable call for the GLEW include path needed to be revised.
Append flags on OSX, rather than overwrite them.
I realized that GCC_COMPILE_FLAGS was changed to CMAKE_CXX_FLAGS mistakenly, so both were changed to a more platform-independent name.

Changes for clarity of comments, removed redundant compiler flags.