In cmake 3.0, it is required to use full path for compilers. And it
seems like there's no need to force the compiler. Instead we can just
use set(CMAKE_C_COMPIER clang) so that we don't need to specify full
path of clang and clang++.

Change-Id: I9f6a625068e9d37a4bf5a595bafd82cbc13fdde4