From the log file, the error says that you need to switch off documentation generation as read from makeinstall file. See this thread SF-1183.
In addition, it seems that the directory in which you installed SALOME leads to PATH variable length which is too long. You might experience PATH length issue at runtime.
You should use something like: C:\S
CMake Error at doc/salome/gui/cmake_install.cmake:40 (file):
file INSTALL cannot find
"C:/Users/kevin.lam/Documents/S/SALOME-9.12.0/BUILD/GUI/doc/salome/gui/GUI":
File exists.
Call Stack (most recent call first):
doc/salome/cmake_install.cmake:38 (include)
doc/cmake_install.cmake:37 (include)
cmake_install.cmake:83 (include)