Add __ANDROID__: some compiler uses this instead of __android__.

[ikeji]

My compiler (clang which provided by termux project) defined __ANDROID__ and does not defined __android__.
This PR addes defined(__ANDROID__) to support these compiler.

Seems like gcc also uses __ANDROID__.
https://gcc.gnu.org/onlinedocs/gcc-4.7.4/gcc/GNU_002fLinux-Options.html

[tsahara]

Can we just replace __android__ with __ANDROID__? All codes I can find in GitHub use __ANDROID__ for Android specific things.

[tsahara]

I found that mruby uses __ANDROID__ (here), so it is better to do the same here. I'll commit the change. Thanks!