Fix ios cmake file for cmake 3.0
Fix ios cmake file for cmake 3.0

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

DougFirErickson/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver