languages.hpp: add support for private_use, parse 'code-x-privateuse' style codes
languages.cpp: map en-uk-{north,rp,wmids} to private use tags (en-GB-x-{lancastrian,rp,westmids})
tests/ttslanguages.cpp: test that all the published language codes are mapped to correct IANA codes
languages.cpp: move mapping quirky eSpeak language codes to espeak.cpp
data/dialects.rdf: rename to private-use.rdf and only keep the iana:Private entries
data/private-use.rdf: merge with languages.rdf using an IANA Subtag Registry style file
tests/languages.cpp: 'en-UK' no longer equates to 'en-GB', so remove that test
mimetype.cpp: detect MIME headers found in mail archives
tests/events.py: create a test for multi-mail archives

This does not add support for multi-part archives yet.

The event list is only a basic event list at the moment. In the
end, this should have a ToC for navigating to each mail in the
archive, and have a basic From/To/Subject/Date header for each
message.
android: initial configure helper for the Android NDK toolchain
android: only use android directories in pkg-config search
autotools: fix detection of zlib on android
android: fix the use of the preprocessor for AC_CHECK_HEADERS
autotools: don't fail configure if pulseaudio is not found (for Android support)
i18n.h: support building when ENABLE_NLS is not defined (Android support)
encoding.cpp: don't fail to build if iconv.h is missing (for Android)
languages.cpp: support building without NLS enabled (e.g. Android)
android: getenv is not supported, so return nullptr instead
pulse.cpp: support building if PulseAudio is not available (for Android)
style_manager.cpp: atoi is defined in stdlib.h (Android build fix)