[Android] Added tools (unix bash scripts) to recompile Android extlibs

For some reason, one might want to recompile the extlibs but this is a tedious task. That's why these scripts are provided.
Please read readme.txt before using them!

tools/android/readme.txt

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+Compiling external libraries for Android can be a tedious task, especially for
+those who aren't familiar with the NDK, that's why we provide these scripts.
+
+IMPORTANT: Please, be careful when using these scripts! They are unpolished at
+the moment and you'll have to respect a simple rule: call these scripts from
+where they are. So, in that case, head yourself to tools/android, then call
+./make_all.sh.
+
+Feel free to improve them or send patches.
+
+HOW-TO-USE:
+-----------
+1) Some of these scripts need an environement variable to work ($NDK) as well
+as the Android CMake toolchain you'll find in cmake/toolchains (android.toolchain.cmake)
+export NDK=/path/to/your/ndk
+export ANDROID_CMAKE_TOOLCHAIN=/path/to/android.toolchain.cmake
+
+2) You'll need to make them executable, so:
+chmod +x *.sh
+
+3) Type: ./make_all.sh which should create standalone toolchains, download the
+external libraries and compile them.
+
+These scripts will be improved over time. Meanwhile, you'll have to play with
+them if you want a customized behavior.
+
+Good luck!