[SOLVED] Failure half-way through build

Hello, I'm trying to build the project for the sake of compiling KiCAD from sources.
I think I followed the instructions well, and make was working for about an hour, and completed 42% of the build, but then ran into an issue.
It looks like an error somewhere, an ld got a bunch of flags where it was expecting a filename, I think.
I'm on Lubuntu 22.04.1, gcc 11, trying to build 7.5.0.
Talk to me like a newbie, though, as I'm an eternal newbie in linux, in spite of living in it for the past 10 years.
Relevant output:

[ 43%] Building CXX object src/TKService/CMakeFiles/TKService.dir/__/Font/Font_FTLibrary.cxx.o
[ 43%] Building CXX object src/TKService/CMakeFiles/TKService.dir/__/Font/Font_SystemFont.cxx.o
[ 43%] Building CXX object src/TKService/CMakeFiles/TKService.dir/__/Font/Font_TextFormatter.cxx.o
[ 43%] Linking CXX shared library ../../lin64/gcc/lib/libTKService.so
/usr/bin/ld: cannot find -lXmu: No such file or directory
collect2: error: ld returned 1 exit status
make[2]: *** [src/TKService/CMakeFiles/TKService.dir/build.make:1621: lin64/gcc/lib/libTKService.so.7.5.0] Error 1
make[1]: *** [CMakeFiles/Makefile2:1723: src/TKService/CMakeFiles/TKService.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

Thanks in advance.