Fix Android target build on Apple hosts.

Currently CMakeLists.txt forces CMAKE_C_COMPILER to clang whenever
compiling on an APPLE host.  Unfortunately, this prevents the
android.toolchain.cmake script from selecting the correct cross
compiler. See [line 1136][].

To avoid this problem, only force clang if ANDROID_NDK is not set.  This
is one of the variables used by Android.mk.

[line 1136]: https://github.com/apitrace/apitrace/blob/master/cmake/toolchain/android.toolchain.cmake#L1136