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)
xmlreader.cpp: strtol is defined in stdlib.h (Android build fix)
espeak.cpp: fix building when eSpeak is not present