CMake: Use check_symbol_exists instead of check_cxx_symbol_exists.

check_cxx_symbol_exists requires CMake 2.8.6, so even though I
recommended it to Owen it's probably better to stay away for now.
This check is not technically correct because we're checking <math.h>
but then using <cmath> in the actual code, but if we run into problems we
can do the same sort of dance as isinf() and isnan() where we check /both/
headers and then write a wrapper header around them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174773 91177308-0d34-0410-b5e6-96231b3b80d8