curl.h: add __ANDROID__ macro check

When working with the Android Standalone Toolchain the compiler defines
this macro:

  /path/to/arm-linux-androideabi-gcc -E -dM - < /dev/null \
  | grep -i android
  #define __ANDROID__ 1

We really need to check both ANDROID and __ANDROID__ since I've observed
that:

* if you use Android.mk file(s) and the 'ndk-build' script (aka vanilla
way), ANDROID is predefined (see -DANDROID extra C flag),

* if you use the Android Standalone Toolchain, then __ANDROID__ is
predefined as stated by the compiler