Add libatomic when needed

[pantierra]

I'm the maintainer of tilemaker in Debian and have recently run into a few problems building tilemaker for the not-so-common ARMel, mips64el, and powerpc architectures:

[100%] Linking CXX executable tilemaker
/usr/bin/cmake -E cmake_link_script CMakeFiles/tilemaker.dir/link.txt --verbose=1
/usr/bin/c++ -g -O2 -ffile-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -DTM_VERSION=2.2.0 -Wdate-time -D_FORTIFY_SOURCE=2 -Wl,-z,relro -Wl,-z,now -rdynamic CMakeFiles/tilemaker.dir/vector_tile.pb.cc.o CMakeFiles/tilemaker.dir/osmformat.pb.cc.o CMakeFiles/tilemaker.dir/src/attribute_store.cpp.o CMakeFiles/tilemaker.dir/src/coordinates.cpp.o CMakeFiles/tilemaker.dir/src/geom.cpp.o CMakeFiles/tilemaker.dir/src/helpers.cpp.o CMakeFiles/tilemaker.dir/src/mbtiles.cpp.o CMakeFiles/tilemaker.dir/src/osm_lua_processing.cpp.o CMakeFiles/tilemaker.dir/src/osm_mem_tiles.cpp.o CMakeFiles/tilemaker.dir/src/osm_store.cpp.o CMakeFiles/tilemaker.dir/src/output_object.cpp.o CMakeFiles/tilemaker.dir/src/pbf_blocks.cpp.o CMakeFiles/tilemaker.dir/src/read_pbf.cpp.o CMakeFiles/tilemaker.dir/src/read_shp.cpp.o CMakeFiles/tilemaker.dir/src/shared_data.cpp.o CMakeFiles/tilemaker.dir/src/shp_mem_tiles.cpp.o CMakeFiles/tilemaker.dir/src/tile_data.cpp.o CMakeFiles/tilemaker.dir/src/tile_worker.cpp.o CMakeFiles/tilemaker.dir/src/tilemaker.cpp.o CMakeFiles/tilemaker.dir/src/write_geometry.cpp.o -o tilemaker  -lprotobuf -lshp -lsqlite3 -llua5.1 -lm -lz -lpthread -ldl /usr/lib/arm-linux-gnueabi/libboost_system.so.1.74.0 /usr/lib/arm-linux-gnueabi/libboost_filesystem.so.1.74.0 /usr/lib/arm-linux-gnueabi/libboost_program_options.so.1.74.0 /usr/lib/arm-linux-gnueabi/libboost_iostreams.so.1.74.0 
/usr/bin/ld: CMakeFiles/tilemaker.dir/src/osm_lua_processing.cpp.o: undefined reference to symbol '__atomic_fetch_sub_8@@LIBATOMIC_1.0'
/usr/bin/ld: /usr/lib/arm-linux-gnueabi/libatomic.so.1: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[3]: *** [CMakeFiles/tilemaker.dir/build.make:428: tilemaker] Error 1
make[3]: Leaving directory '/<<PKGBUILDDIR>>/obj-arm-linux-gnueabi'
make[2]: *** [CMakeFiles/Makefile2:86: CMakeFiles/tilemaker.dir/all] Error 2
make[2]: Leaving directory '/<<PKGBUILDDIR>>/obj-arm-linux-gnueabi'
make[1]: *** [Makefile:139: all] Error 2
make[1]: Leaving directory '/<<PKGBUILDDIR>>/obj-arm-linux-gnueabi'
dh_auto_build: error: cd obj-arm-linux-gnueabi && make -j4 "INSTALL=install --strip-program=true" VERBOSE=1 returned exit code 2
make: *** [debian/rules:16: build-arch] Error 25

These do not support atomic operations, instead relying on emulation provided by libatomic. Therefore, "-latomic" should be added to the command. This PR introduces a check to verify if libatomic needs to be added and adds them if necessary.

[systemed]

Thank you (and sorry for the delay in merging!).