Makefile.system: when Clang is used, check if gfortran is used #2633

Job	Run time
xbuild-x86_64	12m 8s
xbuild-ios	6m 50s
xbuild-ios32	7m 23s
build-windows (cmake, gfortran, 0, 1)	13m 29s
build-windows (make, gfortran, 0, 0)	7m 22s
build-windows (cmake, gfortran, 1, 0)	23m 28s
build-windows (make, gfortran, 0, 1)	7m 11s
build-windows (make, gfortran, 1, 0)	5m 35s
build-windows (cmake, gfortran, 1, 1)	15m 1s
build-windows (cmake, gfortran, 0, 0)	12m 36s
build-windows (make, gfortran, 1, 1)	7m 10s
	1h 58m 13s

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