android-configure.sh: Move OS X SDK detection code up.

This moves the probe for the OS X SDK before the probe of
the host compiler. This is required because the prebuilt Clang
toolchain (under $AOSP/prebuilts/clang/darwin-x86/host/3.5/)
depends on runtime object files like crt1.10.6.o which are
provided by XCode.

As a result, if the user tries to run on a system that doesn't
have XCode installed, the script will now complain with an
appropriate error message, instead of a cryptic linker error like:

  ...
  your linker doesn't seem to work:
  ld: library not found for -lcrt1.10.6.o
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Does not affect non-darwin platforms.

Change-Id: I7fe7d91448e4d991b2483dd1eeca2e50830d73f3
distrib/android-emugl: Move Android GPU emulation sources here.

This patch is used to add the unmodified sources of the GPU
emulation libraries that are normally under sdk/emulator/opengl/.

These sources were originally built with the platform build system,
but this is no longer the case. Since they are only built through
the emulator's own build system (i.e. android-rebuild.sh), it's
easier to put them under external/qemu/ to ensure that they are
always in sync with the rest of the emulator.

Note that the encoder sources are still in the platform under
device/generic/goldfish/opengl/.

A future patch will update the emulator build system to pick
the sources from here, and another one will remove the sources
from sdk/ after that.

Note: these are the unmodified sources from the following
      commit in https://android.googlesource.com/platform/sdk.git :

    182e469 emulator/opengl: Fix Windows cross-build.

Change-Id: If2010577ef4af4c6755ab345f424af0b546f9282
Use GPU emulation sources in distrib/android-emugl/

A previous patch copied the unmodified sources of the GPU emulation
libraries into external/qemu/distrib/android-emugl/, from their
original location in sdk/emulator/opengl/.

This patch modifies the emulator's build system to use the new
sources, and ignore the old one.

+ Update paths in documentation and related scripts.

Change-Id: I8781cc8c2418afa9f2c42f8c53b27086528c6ab8
Merge changes I8781cc8c,If2010577

* changes:
  Use GPU emulation sources in distrib/android-emugl/
  distrib/android-emugl: Move Android GPU emulation sources here.
Merge "android-configure.sh: Move OS X SDK detection code up."