Firstly, you need brew(http://brew.sh) to let it install different packages for compiling.
Once brew is installed, you should install the following packages using terminal:
> brew install cmake
> brew install qt5
> export Qt5_DIR=/usr/local/opt/qt5
Check out the source codes from GitHub or download the ZIP . Go to the directory in Terminal and execute the following commands:
> mkdir build> cd build> cmake -DFORCE_BUNDLED_COPIES=1 -DCMAKE_BUILD_TYPE=Release ..> make
At the end in the folder "bin" is the compiled Sigil.app, which can be dragged into Application folder.