You'll need the following dependencies:

libglib2.0-dev
libgtop2-dev
libgranite-dev
libgtk-3-dev
libwingpanel-2.0-dev
meson
valac

Run meson to configure the build environment and then ninja to build

meson build --prefix=/usr
cd build
ninja

To install, use ninja install

sudo ninja install

Arch Linux users can find Wingpanel Indicator Sys Monitor under the name wingpanel-indicator-sys-monitor-git in the AUR:

$ aurman -S wingpanel-indicator-sys-monitor-git