Broken build after switch to `FetchContent`

[0blu]

After the fix for #171 the cmake build on stock Ubuntu 20.04 and 22.04 is broken.
Using cmake version 3.16.3 (at least on 20.04) (the same issue was reported for 22.04)

git clone https://github.com/jeremy-rifkin/cpptrace
cd cpptrace && mkdir build && cd build
cmake ..

...
-- Cpptrace auto config: Using libgcc unwind for unwinding
-- Cpptrace auto config: Using libdwarf for symbols
-- Cpptrace auto config: Using cxxabi for demangling
CMake Error at /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2555 (message):
  At least one entry of URL is a path (invalid in a list)
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:3236 (_ep_add_download_command)
  CMakeLists.txt:13 (ExternalProject_Add)


-- Configuring incomplete, errors occurred!
See also "/root/cpptrace/build/_deps/zstd-subbuild/CMakeFiles/CMakeOutput.log".

CMake Error at /usr/share/cmake-3.16/Modules/FetchContent.cmake:903 (message):
  CMake step for zstd failed: 1
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/FetchContent.cmake:1006 (__FetchContent_directPopulate)
  /usr/share/cmake-3.16/Modules/FetchContent.cmake:1047 (FetchContent_Populate)
  CMakeLists.txt:429 (FetchContent_MakeAvailable)