Temporary measure to fix a breakage on FreeBSD.
See #551 for more details.

Did this ever work? Surely, doesn't look like it...

Not sure if this ever worked, but now it at least compiles.

This reverts commit 2d6fb5c628068757387525e190c3afdbe33ae9c6,
and adds a fix for Alien::raylib's test failures.

The tests failed because the resulting static library didn't reexport
GLFW symbols. As a fix, we now have GLFW create a CMake "object library"
target that we can link with both the static and shared raylib.

This is arguably ugly... Proper fix would probably be a GLFW upstream
object library target.

Closes #536.

This reverts commit 0adb4b67de6dfe3f7a0a557130dffc3b8f4d6a43,
because it failed tests for Alien::raylib:
http://www.cpantesters.org/distro/A/Alien-raylib.html

Makes it easier to support Wayland later on.

I would have liked config.h to be selected by include dir configuration,
but this way is less intrusive.

If user doesn't build the static library,
`pkg-config --static --libs raylib` should be equivalent to
`pkg-config --libs raylib`.

Fixes this build failure:
http://www.cpantesters.org/cpan/report/a069fade-0e1f-11e8-a1cf-bb670eaac09d

for consistency with Makefile. Requested by @raysan5 in #453.

Let it be noted I utterly dislike their syntax.

-DWITH_SYSTEM_GLFW=ON: Link against system glfw and fail otherwise
-DWITH_SYSTEM_GLFW=OFF: Use embedded rglfw.c
-DWITH_SYSTEM_GLFW=IF_POSSIBLE: Probe for system glfw but fallback to
                                rglfw if unavailable

Also change Linux 64-bit CI build to install system glfw and use it,
so this doesn't bitrot.

Addresses #453.

Removed by mistake.

They were disabled because they failed to build,
but this patch set fixes the build on Linux and macOS.

This doesn't apply to the AppVeyor build on Windows yet;
it currently fails at linking with OpenAL.

After installation, compiling new programs is possible with
    $ cc game.c `pkg-config --static --libs --cflags raylib`
or
    $ cc game.c `pkg-config --libs --cflags raylib`
depending on configuration

Also adds following configuration options:
- WITH_PIC "Compile static library as position-independent code"
- STATIC_RAYLIB "Build raylib as a static library"
- MACOS_FATLIB  "Build fat library for both i386 and x86_64 on macOS"

There have been two problems:

* GLFW itself was compiled with the definitions for compiling
_against_ GLFW (fixed by removing requirement for external glfw)

* rglfw.c was being compiled as C code, although it includes
Objective C files.

This _might_ break the Windows build, needs to be checked.

Fixes #391, but as noted I'd prefer though a separate source directory
and build script for GLFW.

Some people might find this handly