opengl: Simplify emulator build.

This patch simplifies the emulator build configuration to support
OpenGLES emulation. More precisely:

- Remove the include of <libOpenGLRender/render_api.h> and provide
  the corresponding declarations in android/opengles.c in a way
  that make them easy to call indirectly after using dlopen()/dlsym()
  to resolve the symbols.

  This makes the emulator much easier to build as a standalone
  program (it now only depends on external/gtest, and that's only
  for unit tests).

- Fix android-configure.sh to properly detect the location of
  host libraries (it was broken for Windows). Also do not error
  if the libraries cannot be found, since the emulator will simply
  not enable GPU emulation if they are not available at runtime.

- Remove CONFIG_ANDROID_OPENGLES, QEMU_OPENGLES_INCLUDE and
  QEMU_OPENGLES_LIBS from the build, since they are not needed
  anymore.

Change-Id: Ifd300400df5833091e1f06e56d0507de2b973390
8 files changed