cmake: Fix warning about implicit type conversion

Fixes:
CMake Warning (dev) at CMakeLists.txt:478 (set):
  implicitly converting 'String' to 'STRING' type.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMakeLists.txt

@@ -72,7 +72,7 @@ else()
  if(NOT CMAKE_BUILD_TYPE)
  message(STATUS "CMAKE_BUILD_TYPE is not set. Setting default")
  message(STATUS "The available build types are: ${available_build_types}")
- set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE String
+ set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING
  "Options are ${available_build_types}"
  FORCE)
  # Provide drop down menu options in cmake-gui
@@ -475,7 +475,7 @@ set(available_klee_runtime_build_types
 if (NOT KLEE_RUNTIME_BUILD_TYPE)
  message(STATUS "KLEE_RUNTIME_BUILD_TYPE is not set. Setting default")
  message(STATUS "The available runtime build types are: ${available_klee_runtime_build_types}")
- set(KLEE_RUNTIME_BUILD_TYPE "Debug+Asserts" CACHE String
+ set(KLEE_RUNTIME_BUILD_TYPE "Debug+Asserts" CACHE STRING
  "Options are ${available_klee_runtime_build_types}"
  FORCE)
 endif()