[aquaria] CMakeLists.txt - Attempt to use system libraries, or included ones if none available

[Pontos]

Based on the fact that the repository contains pre-compiled libraries 
for OSX and Linux-x86 I only added the corresponding lines for them. 
Should Windows libraries get added in the future then extra code will be 
needed, of course.

I made two different versions of the patch: One that checks and attempts 
to use either the included Mac or Linux libraries respectively if no 
system ones are found, and another that only attempts to use the OSX 
ones, stopping the configuration process if no system libraries for 
Linux are found.
The reason I did that is because even though I looked online I couldn't 
find any way to get the CMake script to differentiate between Linux x86 
and x64, and as such the included libraries would end up being linked 
even when using a x64 OS because of the vague conditional 
if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux").
if(UNIX) should not be used in this case since it works for any 
UNIX-like environment, including Mac, Cygwin and Msys/Mingw.

If any knows a way to make it possible to get the script to recognize 
the architecture of the OS when using Linux then it would be great.

Pontos
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