[flang] Minor revision of the getting started guide.

[c8ef]

• C++17 is now the default, no need to specify explicitly.

llvm-project/flang/CMakeLists.txt

Line 13 in b3c9ed1

set(CMAKE_CXX_STANDARD 17)

• The current recommended way to set up OpenMP is as a runtime, not as a project.

llvm-project/llvm/CMakeLists.txt

Lines 209 to 214 in b3c9ed1

	if ("openmp" IN_LIST LLVM_ENABLE_PROJECTS)
	message(WARNING "Using LLVM_ENABLE_PROJECTS=openmp is deprecated now, and will "
	"become a fatal error in the LLVM 21 release. Please use "
	"-DLLVM_ENABLE_RUNTIMES=openmp or see the instructions at "
	"https://openmp.llvm.org/ for building the runtimes.")
	endif()

[mjklemm]

LGTM, thanks for catching this