Be really strict about dependencies.

The obtained binaries before and after this commit are identical (sha1sum)
when compiled in Debian/Ubuntu/Fedora/ArchLinux.
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The linker will always pick the 32bit libraries the only thing this does is
make sure we have all the 32bit dependencies installed. Basically we avoid
detecting the 64bit libraries and telling the users the 32bit libraries were
found. We always link with 32bit libraries therefore this avoids having to
wait 5-10min to just be told -lXXX is missing.
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The only thing really needed are
set(CMAKE_LIBRARY_ARCHITECTURE "../lib32")
set(CMAKE_LIBRARY_ARCHITECTURE ".")
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which basically ensures we don't pick 64bit headers since
CMAKE_LIBRARY_ARCHITECTURE always gets tested first and for some reason
FindGTK2 test searches lib64 first then lib32/lib. These values are hardcoded.
Right now these arch specific headers are not used but can't say this will
always be true.
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FIND_LIBRARY_USE_LIB64_PATHS is not needed for native builds and it's covered
by CMAKE_SYSTEM_IGNORE_PATH.
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NOTE:
We filter out lib32 because multilib is not compatible with multiarch.
Avoid PKG_CONFIG_PATH issues.

Even before this pull request it was required to set PKG_CONFIG_PATH
when cross compiling since pkg-config always searches for native
libraries. This would require to backup an already existing ENV
variable if present, detect the distro specific place the pkgconfig folder is,
and restore the original ENV variable if it was present. Cmake as shown in:
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http://www.cmake.org/gitweb?p=cmake.git;a=commitdiff;h=3df51470
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added support for this but requires CMAKE_MINIMUM_REQUIRED_VERSION as 3.1
which is kind of strict since it was released 10 days ago.
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To avoid the ugliness let just avoid pkg-config when cross compiling. This
means only EGL, SDL2 and GTK3 have to be updated since the rest work w/o
pkg-config.
- EGL is fixed with PR #409
- SDL2 is fixed here and it's trivial.
- GTK3 is non-trivial. For this one use pkg-config for native builds since
  the .pc file is kept up to date automatically. For cross compilation
  use the provided FindGTK3 which I made by using the sed documented
  inside the file and updating the dependencies for GTK3. Since I checked
  each and every single .pc file to make sure they are up to date this
  should have the same behaviour as pkg-config. Prefer the .pc files for
  native builds since it does not need to be manually updated and when
  the native 64bit port gets finished all the cross compile options should
  be removed which is quite easy since most of the code got moved to the
  CMAKE_TOOLCHAIN_FILE.
Note:
Cmake Modules are BSD-3-clause therefore GPL compatible.
Notify about cross build option upon failure.

Tell the user to use CMAKE_TOOLCHAIN_FILE upon failure.
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Cleanup detectOperatingSystem. Should be the same but adding
GNU and kFreeBSD from Debian and filtering out pre OS X Apple.
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libaio is linux only. Also check for the correct header since
aio.h is the POSIX one. Both are in /usr/include/ anyway.
Only build core if (Linux AND NOT AIO_FOUND) == false.
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Use Unix for GTK and X11. Macs has gtk-quartzs and xquartz or native X11.
*BSD, linux, etc should have both.
Detect clang and automatically set USE_CLANG.

User may forget or not know about -DUSE_CLANG=TRUE. It could probably
be always autodetected instead of requiring user input.
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Trivial wording changes to compiler_version.