src/Makefile.am: reinstate empty FCLAW_CPPFLAGS #703

Job	Run time
CMake build on MacOS	44m 22s
CMake Serial Minimal Build on Linux	58s
CMake Serial With Clawpack Build on Linux	6m 41s
CMake Serial With GeoClaw Build on Linux	1m 12s
CMake Serial With CudaClaw (CUDA 10.1) Build on Linux	3m 18s
CMake Serial With CudaClaw (CUDA 11.3) Build on Linux	3m 54s
CMake MPI Minimal Build on Linux	1m 23s
CMake MPI With Clawpack Build on Linux	7m 8s
CMake MPI With GeoClaw Build on Linux	1m 49s
CMake MPI With CudaClaw (CUDA 10.1) Build on Linux	3m 44s
CMake MPI With CudaClaw (CUDA 11.3) Build on Linux	4m 53s
CMake MPI With ThunderEgg Build on Linux	3m 25s
	1h 22m 47s

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