In addition what Tron wrote.
The example I posted was compiled under Free Pascal IDE.
FPspreadsheet is placed in my user directory in fpc/fpspreadsheet directory and lazutils is under fpc/lazutils directory.
In Free Pascal IDE I added three lines in 'Options/Directories'.
In 'Options/Directories/Units' I added:
../fpc/fpspreadsheet/source/common
../fpc/lazutils
And in 'Options/Directories/Include files' I added:
../fpc/fpspreadsheet/source
Lazarus developers don't want to change C style operators to Pascal in file lazutils/lazutf8.pas, so you have to either turn on compiler option 'C like operators' or change
Result+= to
Result := Result + in 5 places in that file.