build instructions fail

README.txt says "To build locally run `make build`".

When I try that on Ubuntu 24.16, it fails with output:
```
cmake -S . -B build -GNinja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=On -DCPPTRACE_UNWIND_WITH_LIBUNWIND=On
-- Cpptrace auto config: Using libdwarf for symbols
-- Cpptrace auto config: Using cxxabi for demangling
-- ZSTD VERSION: 1.5.6
-- CMAKE_INSTALL_PREFIX: /usr/local
-- CMAKE_INSTALL_LIBDIR: lib
-- ZSTD_LEGACY_SUPPORT not defined!
-- ZSTD_MULTITHREAD_SUPPORT is enabled
-- Building dwarfgen ... OFF
-- Building dwarfexample... OFF
-- Building api tests ...
-- HAVE_UINTPTR_T 1: uintptr_t defined in stdint.h... YES
-- uintptr_t value considered NO
-- HAVE_INTPTR_T 1: intptr_t defined in stdint.h... YES
-- intptr_t value considered NO
-- ENABLE_DECOMPRESSION : ON
-- Found libzstd : TRUE
-- Found zlib : TRUE
-- Build with zlib and zstd: TRUE
-- CMAKE_SIZEOF_VOID_P ... : 8
-- Compiler warning options... NO
-- Install prefix ... /usr/local
-- Configuring done (0.4s)
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
/home/donhatch/gitrepos/glibc-tools/build/_deps/cpptrace-src/LIBUNWIND_INCLUDE_DIRS
used as include directory in directory /home/donhatch/gitrepos/glibc-tools/build/_deps/cpptrace-src

-- Generating done (0.0s)
CMake Generate step failed. Build files cannot be regenerated correctly.
make: *** [Makefile:10: build/configured-debug] Error 1
```