Synchronet supports building with GNU make only. On my Unix/Linux systems, the command to invoke GNU make would be gmake. If the gmake comman doesn't work, try executing make -v and if you see something like the following, then use the make command in place of gmake for building Synchronet.
GNU Make 3.81 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
On some Unixes (e.g. Debian Linux), the GNU make command is make instead of gmake. Creating a symlink or an alias can resolve this issue easily.