Linux README explaining build vs runtime, and build deps for Ubuntu 1…

…3.04

Build dependencies list for Ubuntu 13.04 provided by Ryan.

README-linux.txt

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+================================================================================
+Simple DirectMedia Layer for Linux
+================================================================================
+
+By default SDL will only link against glibc, the rest of the features will be
+enabled dynamically at runtime depending on the available features on the target
+system. So, for example if you built SDL with Xinerama support and the target
+system does not have the Xinerama libraries installed, it will be disabled
+at runtime, and you won't get a missing library error, at least with the 
+default configuration parameters.
+
+
+================================================================================
+Build Dependencies
+================================================================================
+
+Ubuntu 13.04, all available features enabled:
+
+sudo apt-get install build-essential mercurial make cmake autoconf automake \
+libtool libasound2-dev libpulse-dev libaudio-dev libx11-dev libxext-dev \
+libxrandr-dev libxcursor-dev libxi-dev libxinerama-dev libxxf86vm-dev \
+libxss-dev libgl1-mesa-dev libesd0-dev libdbus-1-dev libudev-dev \
+libgles1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev
+
+