configure.ac: give extract-headers.sh as hint if android-headers are missing

Signed-off-by: groleo <adrian.m.negreanu@intel.com>
extract-headers: nitpicks

Signed-off-by: groleo <adrian.m.negreanu@intel.com>
Merge pull request #94 from groleo/tz

configure.ac: give extract-headers.sh as hint if android-headers are missing