Configuring incomplete, errors occurred!

[DroWnThePoor]

cmake -DCMAKE_INSTALL_PREFIX=/usr/local -S . -B build
-- The C compiler identification is GNU 7.5.0
-- The CXX compiler identification is GNU 7.5.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:31 (find_package):
By not providing "FindFFTW3.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "FFTW3", but
CMake did not find one.

Could not find a package configuration file provided by "FFTW3" with any of
the following names:

FFTW3Config.cmake
fftw3-config.cmake

Add the installation prefix of "FFTW3" to CMAKE_PREFIX_PATH or set
"FFTW3_DIR" to a directory containing one of the above files. If "FFTW3"
provides a separate development package or SDK, be sure it has been
installed.

-- Configuring incomplete, errors occurred!
See also "/home/drown/Git/libkeyfinder-2.2.6/CMakeFiles/CMakeOutput.log".

I'm on Ubuntu 18.04, and I have installed fftw3 per the instructions via apt.
I also have the source for fftw3, but haven't tried to install that because it seemed like my distro packages should work.
I have little real knowledge of cmake, but can usually fumble my way through simple builds.
Forgive me if I'm doing something totally backwards.