Makefile generators use wrong relative path for build subdirectories under the source directory but not under the main build directory
cmake version 3.20.0-rc1
When setting `FETCHCONTENT_SOURCE_DIR_<ucName>` using the generators "NMake Makefile" and "Unix Makefile", the generated `CMakeFiles/Makefile2` file mixes up relative path names and absolute path names when referring to the target associated with the set `FETCHCONTENT_SOURCE_DIR_<ucName>`.
This does not happen when using the "Visual Studio 16 2019" generator and building the resulting solution.
See my main project CMakeLists.txt ([rtek_CMakeLists.txt](/uploads/d0c2ada3868a8e49b092bd0c9928f0ae/rtek_CMakeLists.txt)), the dependency CMakeLists.txt ([sdslib_CMakeLists.txt](/uploads/1c3acce14c2a324f4164e14044d22929/sdslib_CMakeLists.txt)) and the generated [Makefile2](/uploads/b37c47243b698583644f9273100f3371/Makefile2) attached.
Error:
```
make[1]: *** No rule to make target '../_deps/sdslib-build/CMakeFiles/sdslib.dir/all', needed by 'CMakeFiles/rtek.dir/all'. Stop.
make: *** [Makefile:91: all] Error 2
```
Relevant part of my file structure:
```
repos/
rtek/
_deps/
build/
CMakeFiles/
Makefile2
CMakeLists.txt
sdslib-cpp/
CMakeLists.txt
```
Configure & build steps:
```
cd repos/rtek
cmake -S. -B./build -G"NMake Makefiles" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DFETCHCONTENT_SOURCE_DIR_SDSLIB="../sdslib-cpp"
cmake --build build
```
- I tried using an absolute path with FETCHCONTENT_SOURCE_DIR_SDSLIB, but that had no effect.
Relevant parts of `Makefile2`:
```
# refers to sdslib.dir\all by relative path name
CMakeFiles\rtek.dir\all: ..\_deps\sdslib-build\CMakeFiles\sdslib.dir\all
# ...
# sdslib.dir\all defined by full path name
C:\Users\stewa\source\repos\rtek\_deps\sdslib-build\CMakeFiles\sdslib.dir\all:
# ...
# sdslib-build/all refers to sdslib.dir/all as absolute path
C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/all: C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/CMakeFiles/sdslib.dir/all
```
Doing the following find-and-replace in `Makefile2` fixes the generated build:
- `C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/CMakeFiles/sdslib.dir/all` -> `../_deps/sdslib-build/CMakeFiles/sdslib.dir/all`
- `C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/all` -> `../_deps/sdslib-build/all`
issue