Feature post compile script xcode

[roymacdonald]

I just added support for having a post compile bash script on xcode on an addons addon_config.mk file.
Among many other things this happens to be really useful when using dynamic libraries, copy these and running install_name_tool. I've seen on lots of addons that some manual addition of a post build script needs to be done, which is cumbersome at least.
The support for adding these scripts was already there but was only used when parsing the projects config.make file.
I decided that ADDON_AFTER_COMPILE_SCRIPTis the flag to be used in addon_config.mk
So for adding a script it can be done on a single line like ADDON_AFTER_COMPILE_SCRIPT = cp "$OF_PATH/addons/ofxNDI/libs/NDI/lib/osx/x64/libndi.3.dylib" "$TARGET_BUILD_DIR/$PRODUCT_NAME.app/Contents/MacOS/"; cd "$TARGET_BUILD_DIR/$PRODUCT_NAME.app/Contents/MacOS/"; install_name_tool -change @rpath/libndi.3.dylib @executable_path/libndi.3.dylib "$PRODUCT_NAME"

or over several lines

ADDON_AFTER_COMPILE_SCRIPT = cp "$OF_PATH/addons/ofxNDI/libs/NDI/lib/osx/x64/libndi.3.dylib" "$TARGET_BUILD_DIR/$PRODUCT_NAME.app/Contents/MacOS/"
ADDON_AFTER_COMPILE_SCRIPT += cd "$TARGET_BUILD_DIR/$PRODUCT_NAME.app/Contents/MacOS/"
ADDON_AFTER_COMPILE_SCRIPT += install_name_tool -change @rpath/libndi.3.dylib @executable_path/libndi.3.dylib "$PRODUCT_NAME"

in #171 makes mention of the flag ADDON_AFTER but no such thing exists