Create an open_memstream compatiblity layer

[rhdunn]

On systems that do not support open_memstream a temporary file is created. While this works, it is slower than using the in-memory version.

BSD-based systems (including Android and Mac OS) have funopen that allows creating custom FILE objects. There are various open_memstream compatibility layers available, e.g.:

1. https://code.google.com/p/tesseract-android-tools/source/browse/tesseract-android-tools/jni/com_googlecode_leptonica_android/stdio/open_memstream.c?r=f5c859a3bd7cd1e1efba38050fb6b79244971df9
2. https://github.com/shyuep/pyhull/tree/master/src/fmemopen

Adapting one of these will allow in-memory file stream support on more platforms (i.e. everything other than Windows).

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