request: replace custom openmp switches with `cmake`'s `find_package(OpenMP)`

[psadil]

This was suggested during review of : conda-forge/staged-recipes#32756 (comment). I'd expect that use of the cmake tools would also make niimath more portable, such as when working on osx with libomp from a different source than homebrew, as this block seems to require

niimath/src/Makefile

Lines 12 to 19 in 4df3c65

	UNAME_S := $(shell uname -s)
	ifeq ($(UNAME_S),Darwin)
	OMPFLAGS= -Xpreprocessor -fopenmp -I/opt/homebrew/opt/libomp/include
	OMPLINK= -L/opt/homebrew/opt/libomp/lib -lomp
	else
	OMPFLAGS= -fopenmp
	OMPLINK= -fopenmp
	endif

Other places that may be simplified with this change:

niimath/SuperBuild/SuperBuild.cmake

Lines 45 to 47 in 4df3c65

	if(${CMAKE_C_COMPILER_ID} STREQUAL "AppleClang")
	option(OPENMP_XCODE "Build with OpenMP support" OFF)
	endif()

niimath/src/CMakeLists.txt

Lines 80 to 82 in 4df3c65

	if(${CMAKE_C_COMPILER_ID} STREQUAL "AppleClang")
	#e.g. cmake -DOPENMP_XCODE=ON ..
	# Default ON when allineate is enabled (benefits from parallel coarse search)