Closes #213.

Apparently, clang defaults to NEON when ARMv7 is used.  Not all ARMv7
CPUs we target have NEON, so we need to disable that.

Android native code should be position-independent.

The NDK build scripts use "-fpic" instead of "-fPIC" for ARM, but that
doesn't work with FFmpeg's assembly code, because it requires
R_ARM_MOVW_ABS_NC which is unavailable with "-fpic".

By telling clang which implementation to use, we avoid the dependency
on libstdc++.so.

.. and make all library symbols hidden by default.

Saves big amounts of .text section size with --gc-sections, because
only this allows discarding unused functions from those (static)
third-party libraries.

Android is migrating away from GCC, and libstdc++ will disappear
eventually.

Support for GCC has been dropped from NDK r13.  Let's switch
everything to clang before they really remove GCC completely.

For Android NDK r13.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

Reduce dependencies on global variables.

Not used anymore.

Fixes a bug that occurs when runing "build.py" from inside the
"android" directory.

Fixes in-tree build when the script is called as "android/build.py"
and not "./android/build.py".

Allow multiple targets to share a library directory tree.

These do not require GLib anymore.