graphics/darktable: fix build on powerpc64le

Use GCC to work around https://github.com/llvm/llvm-project/issues/147288

Add -lexecinfo because of:
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")

if(HAVE_EXECINFO_H)
  add_definitions("-DDT_HAVE_SIGNAL_TRACE")
  message(STATUS "Signal debug: print-trace possible")
endif(HAVE_EXECINFO_H)

endif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")