I also needed to install make.
Anyhow, on my DO droplet I'm getting an error, maybe the installed version of g++ is wrong?
g++ -c -O2 -pthread -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter -g -DBOOST_SPIRIT_THREADSAFE -D_FILE_OFFSET_BITS=64 -I/root/primecoin/src -I/root/primecoin/src/obj -DUSE_UPNP=0 -DUSE_IPV6=1 -I/root/primecoin/src/leveldb/include -I/root/primecoin/src/leveldb/helpers -DHAVE_BUILD_INFO -fno-stack-protector -fstack-protector-all -Wstack-protector -D_FORTIFY_SOURCE=2 -MMD -MF obj/init.d -o obj/init.o init.cpp
g++: internal compiler error: Killed (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-4.7/README.Bugs> for instructions.
make: *** [obj/init.o] Error 4