Fix warnings for CMake Policy CMP0054

Fix warnings for CMake Policy CMP0054

This commit ensures the variables CMAKE_C_COMPILER_ID and
CMAKE_C_COMPILER_ID will always be interpreted by removing
the double quotes.

It fixes warnings like this one:

CMake Warning (dev) at CMakeLists.txt:91 (IF):
  Policy CMP0054 is not set: Only interpret if() arguments as variables or
  keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  Quoted variables like "CMAKE_C_COMPILER_ID" will no longer be dereferenced
  when the policy is set to NEW.  Since the policy is not set the OLD
  behavior will be used.
This warning is for project developers.  Use -Wno-dev to suppress it.