Determining if the include file FLAC++/decoder.h exists failed with the following output:
Change Dir: /var/tmp/portage/kde-apps/kwave-21.04.3/work/kwave-21.04.3_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_8e66f && [1/2] Building CXX object CMakeFiles/cmTC_8e66f.dir/CheckIncludeFile.cxx.o
FAILED: CMakeFiles/cmTC_8e66f.dir/CheckIncludeFile.cxx.o 
/usr/bin/x86_64-pc-linux-gnu-g++ -D_GNU_SOURCE -D_LARGEFILE64_SOURCE   -DQT_NO_DEBUG -pipe -march=native -fno-diagnostics-color -O2 -falign-functions=32:25:16 -fPIC --std=c++11 -fno-operator-names -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -Wvla -Wdate-time -Wsuggest-override -Wlogical-op -fdiagnostics-color=always -o CMakeFiles/cmTC_8e66f.dir/CheckIncludeFile.cxx.o -c /var/tmp/portage/kde-apps/kwave-21.04.3/work/kwave-21.04.3_build/CMakeFiles/CMakeTmp/CheckIncludeFile.cxx
/var/tmp/portage/kde-apps/kwave-21.04.3/work/kwave-21.04.3_build/CMakeFiles/CMakeTmp/CheckIncludeFile.cxx:1:10: fatal error: FLAC++/decoder.h: No such file or directory
    1 | #include <FLAC++/decoder.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.