In Android.mk: skip if cmake is not present

Skip the whole configuration/compilation of apitrace from Android.mk
if cmake is not present in the system.
In Android.mk: explicitly specify the toolchain

If NDK is cloned from git://codeaurora.org/platform/prebuilt instead of
from git://android.git.linaro.org/platform/prebuilt then
prebuilt/linux-x86/toolchain/ contains some extra entries that confuse
the toolchain pickup logic in cmake/toolchain/android.toolchain.cmake,
thus we explicitly specify the toolchain via ANDROID_TOOLCHAIN_NAME.
Merge pull request #131 from vasild/master

Skip if cmake is not available and explicitly specify the toolchain