MSVC lets us fine-tune catching expressions with no side-effects a
little more.

Allows our CI to catch more potential bugs. This also removes the
[[nodiscard]] attribute of IOFile's Open member function. There are
cases where a file may want to be opened, but have the status of it
checked at a later time.

This switch is enabled by default in all recent versions of GCC and
Clang.

* ipc: Allow all trivially copyable objects to be passed directly into WriteBuffer

With the support of C++20, we can use concepts to deduce if a type is an STL container or not.

* More agressive concept for stl containers

* Add -fconcepts

* Move to common namespace

* Add Common::IsBaseOf

Ensures that our code always has its linkage explicit.

Allows catching cases where internal linkage isn't specified for helper
functions when they should be marked as such.

Allows reporting more cases where logic errors may exist, such as
implicit fallthrough cases, etc.

We currently ignore unused parameters, since we currently have many
cases where this is intentional (virtual interfaces).

While we're at it, we can also tidy up any existing code that causes
warnings. This also uncovered a few bugs as well.

This can result in silent logic bugs within code, and given the amount
of times these kind of warnings are caused, they should be flagged at
compile-time so no new code is submitted with them.

This significantly reduces unnecessary disk writes and space usage
when building Citra.

libcore.a is now only ~1MB rather than several hundred megabytes.

By default, MSVC doesn't use standards-compliant volatile semantics.
This makes it behave in a standards-compliant manner, making
expectations more uniform across compilers.

The C++ standard allows constexpr variables declared with the extern
keyword to have external linkage. Previously MSVC wasn't abiding by
this. This just makes the compiler more standards compliant during
builds.

Given we currently don't make use of anything that would break by this,
this is safe to enable.

Makes it much nicer to visually scan the options. This also starts the
flag descriptions from the same column for the same reason.

Ensures that -Wall is always active as a compilation flag.

Previously we were building with MBCS, which is pretty undesirable. We
want the application to be Unicode-aware in general.

Currently, we make the command line variant of yuzu use ANSI variants of
the non-standard getopt functions that we link in for Windows, given we
only have an ANSI option-set.

We should really replace getopt with a library that we make all build
types of yuzu link in, but this will have to do for the time being.

Modifying CMAKE_* related flags directly applies those changes to every
single CMake target. This includes even the targets we have in the
externals directory.

So, if we ever increased our warning levels, or enabled particular ones,
or enabled any other compilation setting, then this would apply to
externals as well, which is often not desirable.

This makes our compilation flag setup less error prone by only applying
our settings to our targets and leaving the externals alone entirely.

This also means we don't end up clobbering any provided flags on the
command line either, allowing users to specifically use the flags they
want.

We generally shouldn't be hijacking CMAKE_CXX_FLAGS, etc as a means to
append flags to the targets, since this adds the compilation flags to
everything, including our externals, which can result in weird issues
and makes the build hierarchy fragile.

Instead, we want to just apply these compilation flags to our targets,
and let those managing external libraries to properly specify their
compilation flags.

This also results in us not getting as many warnings, as we don't raise
the warning level on every external target.

* Added support for network with ENet lib,

connecting is possible, but data can't be sent, yet.

* fixup! Added support for network with ENet lib,

* fixup! CLang

* fixup! Added support for network with ENet lib,

* fixup! Added support for network with ENet lib,

* fixup! Clang format

* More fixups!

* Moved ENetHost* and ENetPeer* into pimpl classes

* fixup! Moved ENetHost* and ENetPeer* into pimpl classes

* fixup! Clang again

* fixup! Moved ENetHost* and ENetPeer* into pimpl classes

* fixup! Moved ENetHost* and ENetPeer* into pimpl classes

* fixup! Moved ENetHost* and ENetPeer* into pimpl classes

citra: Remove GLFW, Add SDL2

FindSDL2: Do not CACHE SDL2_* variables if library is not found

EmuWindow_SDL2: Set minimal client area at initialisation time

EmuWindow_SDL2: Corrections

EmuWindow_SDL2: Fix no decorations on startup on OS X

cmake: windows_copy_files

This commit:
* Adds a new subproject, audio_core.
* Defines structures that exist in DSP shared memory.
* Hooks up various other parts of the emulator into audio core.

This sets the foundation for a later HLE DSP implementation.

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.