To install Otter Browser from source you need to have the Qt SDK (5.15.0 or newer) installed. You can use system-wide libraries when available, but you will need to install header files for modules specified in CMakeLists.txt (listed in the line starting with target_link_libraries). You might also need to install additional header files for GStreamer and libxml2.
In case of errors mentioning QtConcurrent, you may need to ad it to the list of required modules in CMakeLists.txt, by adding it to the line starting with target_link_libraries(otter-browser Qt5::Core, resulting in target_link_libraries(otter-browser Qt5::Core Qt5::Concurrent.
CMake (3.1.0 or newer) is used for the build system. You need to perform these steps to get clean build (last step is optional):
mkdir build
cd build
cmake ../
make
make install
Alternatively you can use either Qt Creator to compile sources or export native project files using CMake generators. You can also use CPack to create packages.
To make a portable version of Otter, create a file named arguments.txt with this line:
--portable
Place this file in the directory containing the main Otter executable (the file with name starting with otter-browser).