Add missing X11 extension headers.

This patch modifies the set of packages used to build the toolchain
sysroot to include libxext-dev, which is required to provide certain
headers like <X11/extensions/XShm.h> required to build the emulator's
SDL sources.

+ Ensure the environment is properly sanitized when building the

Change-Id: I219d647bae8e71b353b5973261150f4bb6656501
23 files changed