configure.ac: Add support for Android builds

configure.ac: Add support for Android builds

Add support for EGL android platform.

Also, detect when --host finishes with -android. In that case, we
do not set _GNU_SOURCE, and define autoconf symbol HAVE_ANDROID, so
that Android-specific workarounds can be applied.

Signed-off-by: Nicolas Boichat <drinkcat@google.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
[Emil Velikov: Rebase on top of HAVE_EGL_PLATFORM_NULL removal]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>