Update depfile generation for device compiler

Ninja has a problem if the name of the object doesn't match what it expects, so this changes the generation to output a relative path, not an absolute one. Additionally, the quotes in the optional DEPFILE variable were causing problems, so they have also been removed. Many thanks to @jwlawson for finding and fixing this.

Author: DuncanMcBain

cmake/Modules/FindComputeCpp.cmake

@@ -286,8 +286,9 @@ function(__build_ir)
  # CMake throws an error if it is unsupported by the generator (i. e. not ninja)
  if((NOT CMAKE_VERSION VERSION_LESS 3.7.0) AND
  CMAKE_GENERATOR MATCHES "Ninja")
- set(generate_depfile -MMD -MF ${depFileName})
- set(enable_depfile "DEPFILE ${depFileName}")
+ file(RELATIVE_PATH relOutputFile ${CMAKE_BINARY_DIR} ${outputSyclFile})
+ set(generate_depfile -MMD -MF ${depFileName} -MT ${relOutputFile})
+ set(enable_depfile DEPFILE ${depFileName})
  endif()
 
  # Add custom command for running compute++